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Systematic reviews: Structure, form and content

This article aims to provide an overview of the structure, form and content of systematic reviews. It focuses in particular on the literature searching component, and covers systematic database searching techniques, searching for grey literature and the importance of librarian involvement in the search. It also covers systematic review reporting standards such as PRISMA-P and PRISMA, critical appraisal and tools and resources to support the review and ensure it is conducted efficiently and effectively. Finally, it summarizes the requirements when screening search results for inclusion in the review, and the statistical synthesis of included studies’ findings.

Provenance and Peer review: Solicited contribution; Peer reviewed; Accepted for publication 24 January 2021.

Introduction

A systematic review collects secondary data, and is a synthesis of all available, relevant evidence which brings together all existing primary studies for review ( Cochrane 2016 ). A systematic review differs from other types of literature review in several major ways. It requires a transparent, reproducible methodology which indicates how studies were identified and the criteria upon which they were included or excluded. As well as synthesis of these studies' findings, there should be an element of evaluation and quality assessment. The systematic review methodology originated in medical and healthcare research, but it has now been adopted by other disciplines, such as engineering, education, economics and business studies. The processes and requirements for conducting a systematic review can seem arduous or time consuming, but with the use of appropriate tools and resources, and with thorough planning undertaken before beginning the review, researchers will be able to conduct their systematic reviews efficiently and smoothly.

This article provides an overview of the structure, form and content of systematic reviews, with a particular focus on the literature searching component. It will also discuss tools and resources – including those relating to reporting standards and critical appraisal of the articles included in the review – which will be of use to researchers conducting a systematic review.

Topic selection and planning

In recent years, there has been an explosion in the number of systematic reviews conducted and published ( Chalmers & Fox 2016 , Fontelo & Liu 2018 , Page et al 2015 ) – although a systematic review may be an inappropriate or unnecessary research methodology for answering many research questions. Systematic reviews can be inadvisable for a variety of reasons. It may be that the topic is too new and there are not enough relevant published papers to synthesise and analyse for a systematic review, or, conversely, that many other researchers have already published systematic reviews on the topic. However, if a scoping search appears to yield sufficient relevant studies for evidence synthesis, and indicates that no previous systematic reviews have been published (or that those previously published require an update or have methodological flaws), systematic reviews are likely to be appropriate.

Most systematic reviews take between six and 18 months to complete, and require a minimum of three authors to independently screen search results. Although many university modules require students to complete systematic reviews, due to this time and authorship requirement, it would be better to describe such student reviews as ‘reviews with systematic literature searches,’ as it is not possible to fulfil all the methodological requirements of a systematic review in a piece of work with a single author. Researchers without the available time or number of potential co-authors may prefer to adopt a different approach, such as narrative, scoping, or umbrella reviews. The systematic, transparent searching techniques outlined in this article can be adopted and adapted for use in other forms of literature review ( Grant & Booth 2009 ), for example, while the critical appraisal tools highlighted are appropriate for use in other contexts in which the reliability and applicability of medical research require evaluation.

Once it has been determined that a systematic review is the appropriate methodology for the research, and that there is sufficient time and resources to conduct it, researchers should then spend some time developing their review topic. It is appropriate at this point to do some scoping searches in relevant subject databases, first to ensure that the proposed review is unique, and meets a research need, and second to obtain a broad overview of the literature that exists, and which is likely to be included in the eventual systematic review. Based on this scoping work, the review topic may need to be refined or adapted, possibly to broaden or narrow it in focus. Once reviewers are satisfied with their chosen topic, the next step is to prepare a protocol which states transparently the methodology they intend to follow when conducting their review.

Creating a protocol

A protocol is a description of the proposed systematic review, including methods, the rationale for the review, and steps which will be taken to eliminate bias while conducting the review. Registering the protocol stakes a claim on the research, and it also means that researchers have done a significant portion of the work required before they formally begin the review, as they will have written the Methods section in draft form and planned what will be necessary to document and report by the time the protocol is finished.

Most protocols are registered with PROSPERO (2020), although it is also possible to upload your protocol on an institutional or subject repository, or publish the protocol in a journal. Guidance for creating a protocol can be found at PRISMA-P (The PRISMA Group et al 2015), or by working through the online training on protocols available at the Cochrane Library ( Cochrane Interactive Learning 2019 ).

Reporting standards and structure

PRISMA (the Preferred Reporting Items for Systematic Reviews and Meta-analyses) is 'an evidence-based minimum set of items for reporting in systematic reviews and meta-analyses' ( Moher et al 2009 ). The PRISMA checklist is a useful guideline of content that should be reported and included in the final published version of the systematic review, and will help when in the planning stages as well. Most systematic reviews will be written up using the PRISMA checklist as their underlying structure, so familiarity with this checklist and the content required when reporting the findings of the systematic review should be established at the earliest planning stages of the research.

PRISMA-P (The PRISMA Group et al 2015) is the reporting guidelines for protocols. The EQUATOR Network lists reporting standards for multiple different types of study design ( EQUATOR Network 2020 ). Researchers can search for the right guideline for their type of study. Those undertaking a Cochrane review should select the correct Cochrane Handbook ( Cochrane Training 2020 ) for their review type.

Search strategy

The search strategy for systematic reviews is the main method of collecting the data which will underpin the review's findings. This means that the search must be sufficiently robust – both sensitive and specific – to capture all relevant articles. Ideally, multiple databases and other sources of information should be searched, using a consistent, predetermined search string. Generally, this will involve multiple synonyms for each theme of the review's topic, and a multifield search including freetext terms in (at minimum) the title and abstract, and the controlled vocabulary in the database thesaurus. These words are then combined with the Boolean operators AND, OR and NOT so that search results are both sensitive and specific.

Grey literature

It is likely that systematic reviews will need to include a search of grey literature as well as the peer-reviewed journal articles found through database searching. Grey literature includes unpublished theses, conference proceedings, government reports, unpublished trial data and more. Leaving grey literature out can run the risk of biasing the reviews results ( Goldacre 2011 ).

Searching grey literature can be challenging. Most sources of grey literature cannot be searched with complex Boolean operators and myriad synonymous keywords in the manner of a database. Likewise, the websites and other sources used to search for grey literature are unlikely to have a controlled vocabulary thesaurus. The Canadian Agency for Drugs and Technologies in Health (CADTH) tool is designed to help adapt complex systematic database search strategies for use when searching for grey literature ( CADTH 2009 ).

Snowballing, hand-searching and reference lists

Sometimes it may be appropriate to 'snowball' a search. This involves screening all the articles that cite included papers (the articles which meet the inclusion criteria after screening). Search for the titles of each included article in Web of Science or Scopus (or both), and any listed citing article which meets your inclusion criteria should also be included in the review.

Hand searching involves looking back through the tables of contents of key journals, conference proceedings, or lists of conference presentations relevant to the systematic review topic. Once key journals have been identified, reviewers should plan how many years back they will look – this will need to be done consistently across all journals that are hand-searched.

After reviewers have screened all the papers identified by the database and grey literature searches, and agreed on which will be included in the review, they should check through these articles' reference lists. Any articles in their reference lists which meet all inclusion criteria should also be included in the review.

Librarian co-authorship

There is some evidence that having a librarian co-author on a systematic review can improve the review's quality. A number of recent studies have indicated that librarian involvement improves the reproducibility of the literature searching ( Hameed et al 2020 , Koffel 2015 , Rethlefsen et al 2015 ). Reviews without librarian involvement often have problems with their search strategies – for example Boolean operators used incorrectly, inappropriate search syntax, or a lack of sufficient synonyms for each search term, meaning that relevant studies might be missed ( Golder et al 2008 , Li et al 2014 ). Unfortunately, in some instances, systematic reviews without librarian co-authors will still be published, even if their search strategies have significant methodological flaws ( Brasher & Giustini 2020 ). Librarian involvement will help ensure that the search strategy is robust, and that it is described accurately in the methodology to ensure that the systematic review is reproducible. Generally, if a librarian is developing the search terms, running the searches in databases and writing the search methods, they should be a co-author of the systematic review, whereas if the librarian supports researchers who then conduct the searches themselves, co-authorship is not necessary. This also aligns with the Vancouver recommendations on co-authorship ( International Committee of Medical Journal Editors 2019 ).

After database and grey literature searches are completed, and researchers have identified other papers through hand-searching, they will need to screen the titles and abstracts to determine if they meet the criteria for inclusion. These criteria should be pre-defined (ie: stated in the protocol before searches have begun). Inclusion criteria might relate to the following:

Date range of publication. Study design type. Whether a study focuses on the review's specific disease, condition, or patient population. Whether a study focuses mainly on the review's specific intervention. Whether a study focused on a certain country, region, or healthcare context (for example primary care, outpatient department, critical care unit, or similar).

This list is not exhaustive, and there are many other inclusion criteria to apply, depending on the scope of the topic of the systematic review. It is important that these criteria are stated clearly in the Methods section of both the protocol and systematic review, and that all co-authors understand them.

Generally, articles are screened against these criteria independently by at least two authors. Initially they should screen the titles and abstracts, and then move on to screening the full text for any articles which could not be judged as fulfilling (or not fulfilling) all inclusion criteria on the basis of the information in their titles and abstracts.

Referencing software such as Endnote, EndnoteWeb, Mendeley or Zotero can be used for screening, or reviewers may prefer to use systematic review screening software such as Covidence or Rayyan.

Critical appraisal tools

There are a number of tools and checklists available to help assess the quality of studies to be included in a review. Studies included in a systematic review should be assessed for their quality and reliability. While poor quality studies should not be excluded if they fulfil predefined inclusion criteria, the systematic review should make clear that all included studies have been assessed according to consistent principles of critical appraisal, and the results of that appraisal should be included in the review.

Most critical appraisal tools consist of different checklists to apply to different types of study design. If a systematic review includes multiple types of study design, it is advisable that researchers are consistent about which tools they use – it is preferable to use different checklists from a single source, rather than picking and choosing from a variety of sources.

If the systematic review is only including peer-reviewed, published journal articles, the checklists from either CASP (Critical Appraisal Skills Programme), Centre for Evidence-Based Medicine, SIGN (Scottish Intercollegiate Guidelines Network), or Joanna Briggs Institute will be appropriate ( Brice 2020 , Centre for Evidence-Based Medicine 2020 , Joanna Briggs Institute 2020 , SIGN 2020 ). Reviews which include grey literature should use a grey literature appraisal tool, such as AACODS ( Tyndall 2008 ). There are also risk of bias assessment tools, such as RoBiS for evaluating systematic reviews, and RoB 2 for evaluating randomized controlled trials ( Bristol Medical School 2020 , Sterne et al 2019 ).

One of the main advantages of systematic reviews is that they combine the analysis of the data from a number of primary studies. Most commonly, this is done through meta-analysis – the statistical combination of results from two or more studies. As outlined in the Cochrane Handbook, in interventional studies, a systematic review meta-analysis will seek to answer these three main questions:

What is the direction of effect? What is the size of effect? Is the effect consistent across [all included] studies? ( Higgins et al 2019 )

The researchers will then make a judgement as to the strength of evidence for the effect. If the systematic review is assessing the effectiveness of a variety of different interventions, it may not be possible to combine all studies for meta-analysis as the studies may be sufficiently different to make meta-analysis inappropriate. Researchers should ensure that when interpreting the results they consider the limitations and potential biases of included studies. When reporting the findings it is also usually necessary to consider applicability, and make recommendations – such as for a change in practice.

Systematic reviews – when an appropriate approach to the topic being researched – are a way to synthesize and evaluate the range of evidence available in multiple primary studies. Their methodology is complex, but if the correct reporting guidelines are followed, and researchers make use of tools, resources and the support of librarians and other information specialists, the process will be more straightforward. Planning is key: researchers should have a clear picture of what is involved, and what will need to be documented and reported in any resulting publications, and put measures in place to ensure that they capture all of this essential information.

No competing interests declared .

ORCID iD: Veronica Phillips https://orcid.org/0000-0002-4383-9434

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The PRISMA 2020 statement: an updated guideline for reporting systematic reviews

PRISMA 2020 explanation and elaboration: updated guidance and exemplars for reporting systematic reviews

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• Peer review
• Matthew J Page , senior research fellow 1 ,
• Joanne E McKenzie , associate professor 1 ,
• Patrick M Bossuyt , professor 2 ,
• Isabelle Boutron , professor 3 ,
• Tammy C Hoffmann , professor 4 ,
• Cynthia D Mulrow , professor 5 ,
• Larissa Shamseer , doctoral student 6 ,
• Jennifer M Tetzlaff , research product specialist 7 ,
• Elie A Akl , professor 8 ,
• Sue E Brennan , senior research fellow 1 ,
• Roger Chou , professor 9 ,
• Julie Glanville , associate director 10 ,
• Jeremy M Grimshaw , professor 11 ,
• Asbjørn Hróbjartsson , professor 12 ,
• Manoj M Lalu , associate scientist and assistant professor 13 ,
• Tianjing Li , associate professor 14 ,
• Elizabeth W Loder , professor 15 ,
• Evan Mayo-Wilson , associate professor 16 ,
• Steve McDonald , senior research fellow 1 ,
• Luke A McGuinness , research associate 17 ,
• Lesley A Stewart , professor and director 18 ,
• James Thomas , professor 19 ,
• Andrea C Tricco , scientist and associate professor 20 ,
• Vivian A Welch , associate professor 21 ,
• Penny Whiting , associate professor 17 ,
• David Moher , director and professor 22
• 1 School of Public Health and Preventive Medicine, Monash University, Melbourne, Australia
• 2 Department of Clinical Epidemiology, Biostatistics and Bioinformatics, Amsterdam University Medical Centres, University of Amsterdam, Amsterdam, Netherlands
• 3 Université de Paris, Centre of Epidemiology and Statistics (CRESS), Inserm, F 75004 Paris, France
• 4 Institute for Evidence-Based Healthcare, Faculty of Health Sciences and Medicine, Bond University, Gold Coast, Australia
• 5 University of Texas Health Science Center at San Antonio, San Antonio, Texas, USA; Annals of Internal Medicine
• 6 Knowledge Translation Program, Li Ka Shing Knowledge Institute, Toronto, Canada; School of Epidemiology and Public Health, Faculty of Medicine, University of Ottawa, Ottawa, Canada
• 7 Evidence Partners, Ottawa, Canada
• 8 Clinical Research Institute, American University of Beirut, Beirut, Lebanon; Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada
• 9 Department of Medical Informatics and Clinical Epidemiology, Oregon Health & Science University, Portland, Oregon, USA
• 10 York Health Economics Consortium (YHEC Ltd), University of York, York, UK
• 11 Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Canada; School of Epidemiology and Public Health, University of Ottawa, Ottawa, Canada; Department of Medicine, University of Ottawa, Ottawa, Canada
• 12 Centre for Evidence-Based Medicine Odense (CEBMO) and Cochrane Denmark, Department of Clinical Research, University of Southern Denmark, Odense, Denmark; Open Patient data Exploratory Network (OPEN), Odense University Hospital, Odense, Denmark
• 13 Department of Anesthesiology and Pain Medicine, The Ottawa Hospital, Ottawa, Canada; Clinical Epidemiology Program, Blueprint Translational Research Group, Ottawa Hospital Research Institute, Ottawa, Canada; Regenerative Medicine Program, Ottawa Hospital Research Institute, Ottawa, Canada
• 14 Department of Ophthalmology, School of Medicine, University of Colorado Denver, Denver, Colorado, United States; Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA
• 15 Division of Headache, Department of Neurology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA; Head of Research, The BMJ , London, UK
• 16 Department of Epidemiology and Biostatistics, Indiana University School of Public Health-Bloomington, Bloomington, Indiana, USA
• 17 Population Health Sciences, Bristol Medical School, University of Bristol, Bristol, UK
• 18 Centre for Reviews and Dissemination, University of York, York, UK
• 19 EPPI-Centre, UCL Social Research Institute, University College London, London, UK
• 20 Li Ka Shing Knowledge Institute of St. Michael's Hospital, Unity Health Toronto, Toronto, Canada; Epidemiology Division of the Dalla Lana School of Public Health and the Institute of Health Management, Policy, and Evaluation, University of Toronto, Toronto, Canada; Queen's Collaboration for Health Care Quality Joanna Briggs Institute Centre of Excellence, Queen's University, Kingston, Canada
• 21 Methods Centre, Bruyère Research Institute, Ottawa, Ontario, Canada; School of Epidemiology and Public Health, Faculty of Medicine, University of Ottawa, Ottawa, Canada
• 22 Centre for Journalology, Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Canada; School of Epidemiology and Public Health, Faculty of Medicine, University of Ottawa, Ottawa, Canada
• Correspondence to: M J Page matthew.page{at}monash.edu
• Accepted 4 January 2021

The Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement, published in 2009, was designed to help systematic reviewers transparently report why the review was done, what the authors did, and what they found. Over the past decade, advances in systematic review methodology and terminology have necessitated an update to the guideline. The PRISMA 2020 statement replaces the 2009 statement and includes new reporting guidance that reflects advances in methods to identify, select, appraise, and synthesise studies. The structure and presentation of the items have been modified to facilitate implementation. In this article, we present the PRISMA 2020 27-item checklist, an expanded checklist that details reporting recommendations for each item, the PRISMA 2020 abstract checklist, and the revised flow diagrams for original and updated reviews.

Systematic reviews serve many critical roles. They can provide syntheses of the state of knowledge in a field, from which future research priorities can be identified; they can address questions that otherwise could not be answered by individual studies; they can identify problems in primary research that should be rectified in future studies; and they can generate or evaluate theories about how or why phenomena occur. Systematic reviews therefore generate various types of knowledge for different users of reviews (such as patients, healthcare providers, researchers, and policy makers). 1 2 To ensure a systematic review is valuable to users, authors should prepare a transparent, complete, and accurate account of why the review was done, what they did (such as how studies were identified and selected) and what they found (such as characteristics of contributing studies and results of meta-analyses). Up-to-date reporting guidance facilitates authors achieving this. 3

The Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement published in 2009 (hereafter referred to as PRISMA 2009) 4 5 6 7 8 9 10 is a reporting guideline designed to address poor reporting of systematic reviews. 11 The PRISMA 2009 statement comprised a checklist of 27 items recommended for reporting in systematic reviews and an “explanation and elaboration” paper 12 13 14 15 16 providing additional reporting guidance for each item, along with exemplars of reporting. The recommendations have been widely endorsed and adopted, as evidenced by its co-publication in multiple journals, citation in over 60 000 reports (Scopus, August 2020), endorsement from almost 200 journals and systematic review organisations, and adoption in various disciplines. Evidence from observational studies suggests that use of the PRISMA 2009 statement is associated with more complete reporting of systematic reviews, 17 18 19 20 although more could be done to improve adherence to the guideline. 21

Many innovations in the conduct of systematic reviews have occurred since publication of the PRISMA 2009 statement. For example, technological advances have enabled the use of natural language processing and machine learning to identify relevant evidence, 22 23 24 methods have been proposed to synthesise and present findings when meta-analysis is not possible or appropriate, 25 26 27 and new methods have been developed to assess the risk of bias in results of included studies. 28 29 Evidence on sources of bias in systematic reviews has accrued, culminating in the development of new tools to appraise the conduct of systematic reviews. 30 31 Terminology used to describe particular review processes has also evolved, as in the shift from assessing “quality” to assessing “certainty” in the body of evidence. 32 In addition, the publishing landscape has transformed, with multiple avenues now available for registering and disseminating systematic review protocols, 33 34 disseminating reports of systematic reviews, and sharing data and materials, such as preprint servers and publicly accessible repositories. To capture these advances in the reporting of systematic reviews necessitated an update to the PRISMA 2009 statement.

Summary points

To ensure a systematic review is valuable to users, authors should prepare a transparent, complete, and accurate account of why the review was done, what they did, and what they found

The PRISMA 2020 statement provides updated reporting guidance for systematic reviews that reflects advances in methods to identify, select, appraise, and synthesise studies

The PRISMA 2020 statement consists of a 27-item checklist, an expanded checklist that details reporting recommendations for each item, the PRISMA 2020 abstract checklist, and revised flow diagrams for original and updated reviews

We anticipate that the PRISMA 2020 statement will benefit authors, editors, and peer reviewers of systematic reviews, and different users of reviews, including guideline developers, policy makers, healthcare providers, patients, and other stakeholders

Development of PRISMA 2020

A complete description of the methods used to develop PRISMA 2020 is available elsewhere. 35 We identified PRISMA 2009 items that were often reported incompletely by examining the results of studies investigating the transparency of reporting of published reviews. 17 21 36 37 We identified possible modifications to the PRISMA 2009 statement by reviewing 60 documents providing reporting guidance for systematic reviews (including reporting guidelines, handbooks, tools, and meta-research studies). 38 These reviews of the literature were used to inform the content of a survey with suggested possible modifications to the 27 items in PRISMA 2009 and possible additional items. Respondents were asked whether they believed we should keep each PRISMA 2009 item as is, modify it, or remove it, and whether we should add each additional item. Systematic review methodologists and journal editors were invited to complete the online survey (110 of 220 invited responded). We discussed proposed content and wording of the PRISMA 2020 statement, as informed by the review and survey results, at a 21-member, two-day, in-person meeting in September 2018 in Edinburgh, Scotland. Throughout 2019 and 2020, we circulated an initial draft and five revisions of the checklist and explanation and elaboration paper to co-authors for feedback. In April 2020, we invited 22 systematic reviewers who had expressed interest in providing feedback on the PRISMA 2020 checklist to share their views (via an online survey) on the layout and terminology used in a preliminary version of the checklist. Feedback was received from 15 individuals and considered by the first author, and any revisions deemed necessary were incorporated before the final version was approved and endorsed by all co-authors.

The PRISMA 2020 statement

Scope of the guideline.

The PRISMA 2020 statement has been designed primarily for systematic reviews of studies that evaluate the effects of health interventions, irrespective of the design of the included studies. However, the checklist items are applicable to reports of systematic reviews evaluating other interventions (such as social or educational interventions), and many items are applicable to systematic reviews with objectives other than evaluating interventions (such as evaluating aetiology, prevalence, or prognosis). PRISMA 2020 is intended for use in systematic reviews that include synthesis (such as pairwise meta-analysis or other statistical synthesis methods) or do not include synthesis (for example, because only one eligible study is identified). The PRISMA 2020 items are relevant for mixed-methods systematic reviews (which include quantitative and qualitative studies), but reporting guidelines addressing the presentation and synthesis of qualitative data should also be consulted. 39 40 PRISMA 2020 can be used for original systematic reviews, updated systematic reviews, or continually updated (“living”) systematic reviews. However, for updated and living systematic reviews, there may be some additional considerations that need to be addressed. Where there is relevant content from other reporting guidelines, we reference these guidelines within the items in the explanation and elaboration paper 41 (such as PRISMA-Search 42 in items 6 and 7, Synthesis without meta-analysis (SWiM) reporting guideline 27 in item 13d). Box 1 includes a glossary of terms used throughout the PRISMA 2020 statement.

Glossary of terms

Systematic review —A review that uses explicit, systematic methods to collate and synthesise findings of studies that address a clearly formulated question 43

Statistical synthesis —The combination of quantitative results of two or more studies. This encompasses meta-analysis of effect estimates (described below) and other methods, such as combining P values, calculating the range and distribution of observed effects, and vote counting based on the direction of effect (see McKenzie and Brennan 25 for a description of each method)

Meta-analysis of effect estimates —A statistical technique used to synthesise results when study effect estimates and their variances are available, yielding a quantitative summary of results 25

Outcome —An event or measurement collected for participants in a study (such as quality of life, mortality)

Result —The combination of a point estimate (such as a mean difference, risk ratio, or proportion) and a measure of its precision (such as a confidence/credible interval) for a particular outcome

Report —A document (paper or electronic) supplying information about a particular study. It could be a journal article, preprint, conference abstract, study register entry, clinical study report, dissertation, unpublished manuscript, government report, or any other document providing relevant information

Record —The title or abstract (or both) of a report indexed in a database or website (such as a title or abstract for an article indexed in Medline). Records that refer to the same report (such as the same journal article) are “duplicates”; however, records that refer to reports that are merely similar (such as a similar abstract submitted to two different conferences) should be considered unique.

Study —An investigation, such as a clinical trial, that includes a defined group of participants and one or more interventions and outcomes. A “study” might have multiple reports. For example, reports could include the protocol, statistical analysis plan, baseline characteristics, results for the primary outcome, results for harms, results for secondary outcomes, and results for additional mediator and moderator analyses

PRISMA 2020 is not intended to guide systematic review conduct, for which comprehensive resources are available. 43 44 45 46 However, familiarity with PRISMA 2020 is useful when planning and conducting systematic reviews to ensure that all recommended information is captured. PRISMA 2020 should not be used to assess the conduct or methodological quality of systematic reviews; other tools exist for this purpose. 30 31 Furthermore, PRISMA 2020 is not intended to inform the reporting of systematic review protocols, for which a separate statement is available (PRISMA for Protocols (PRISMA-P) 2015 statement 47 48 ). Finally, extensions to the PRISMA 2009 statement have been developed to guide reporting of network meta-analyses, 49 meta-analyses of individual participant data, 50 systematic reviews of harms, 51 systematic reviews of diagnostic test accuracy studies, 52 and scoping reviews 53 ; for these types of reviews we recommend authors report their review in accordance with the recommendations in PRISMA 2020 along with the guidance specific to the extension.

How to use PRISMA 2020

The PRISMA 2020 statement (including the checklists, explanation and elaboration, and flow diagram) replaces the PRISMA 2009 statement, which should no longer be used. Box 2 summarises noteworthy changes from the PRISMA 2009 statement. The PRISMA 2020 checklist includes seven sections with 27 items, some of which include sub-items ( table 1 ). A checklist for journal and conference abstracts for systematic reviews is included in PRISMA 2020. This abstract checklist is an update of the 2013 PRISMA for Abstracts statement, 54 reflecting new and modified content in PRISMA 2020 ( table 2 ). A template PRISMA flow diagram is provided, which can be modified depending on whether the systematic review is original or updated ( fig 1 ).

Noteworthy changes to the PRISMA 2009 statement

Inclusion of the abstract reporting checklist within PRISMA 2020 (see item #2 and table 2 ).

Movement of the ‘Protocol and registration’ item from the start of the Methods section of the checklist to a new Other section, with addition of a sub-item recommending authors describe amendments to information provided at registration or in the protocol (see item #24a-24c).

Modification of the ‘Search’ item to recommend authors present full search strategies for all databases, registers and websites searched, not just at least one database (see item #7).

Modification of the ‘Study selection’ item in the Methods section to emphasise the reporting of how many reviewers screened each record and each report retrieved, whether they worked independently, and if applicable, details of automation tools used in the process (see item #8).

Addition of a sub-item to the ‘Data items’ item recommending authors report how outcomes were defined, which results were sought, and methods for selecting a subset of results from included studies (see item #10a).

Splitting of the ‘Synthesis of results’ item in the Methods section into six sub-items recommending authors describe: the processes used to decide which studies were eligible for each synthesis; any methods required to prepare the data for synthesis; any methods used to tabulate or visually display results of individual studies and syntheses; any methods used to synthesise results; any methods used to explore possible causes of heterogeneity among study results (such as subgroup analysis, meta-regression); and any sensitivity analyses used to assess robustness of the synthesised results (see item #13a-13f).

Addition of a sub-item to the ‘Study selection’ item in the Results section recommending authors cite studies that might appear to meet the inclusion criteria, but which were excluded, and explain why they were excluded (see item #16b).

Splitting of the ‘Synthesis of results’ item in the Results section into four sub-items recommending authors: briefly summarise the characteristics and risk of bias among studies contributing to the synthesis; present results of all statistical syntheses conducted; present results of any investigations of possible causes of heterogeneity among study results; and present results of any sensitivity analyses (see item #20a-20d).

Addition of new items recommending authors report methods for and results of an assessment of certainty (or confidence) in the body of evidence for an outcome (see items #15 and #22).

Addition of a new item recommending authors declare any competing interests (see item #26).

Addition of a new item recommending authors indicate whether data, analytic code and other materials used in the review are publicly available and if so, where they can be found (see item #27).

PRISMA 2020 item checklist

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PRISMA 2020 for Abstracts checklist*

PRISMA 2020 flow diagram template for systematic reviews. The new design is adapted from flow diagrams proposed by Boers, 55 Mayo-Wilson et al. 56 and Stovold et al. 57 The boxes in grey should only be completed if applicable; otherwise they should be removed from the flow diagram. Note that a “report” could be a journal article, preprint, conference abstract, study register entry, clinical study report, dissertation, unpublished manuscript, government report or any other document providing relevant information.

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We recommend authors refer to PRISMA 2020 early in the writing process, because prospective consideration of the items may help to ensure that all the items are addressed. To help keep track of which items have been reported, the PRISMA statement website ( http://www.prisma-statement.org/ ) includes fillable templates of the checklists to download and complete (also available in the data supplement on bmj.com). We have also created a web application that allows users to complete the checklist via a user-friendly interface 58 (available at https://prisma.shinyapps.io/checklist/ and adapted from the Transparency Checklist app 59 ). The completed checklist can be exported to Word or PDF. Editable templates of the flow diagram can also be downloaded from the PRISMA statement website.

We have prepared an updated explanation and elaboration paper, in which we explain why reporting of each item is recommended and present bullet points that detail the reporting recommendations (which we refer to as elements). 41 The bullet-point structure is new to PRISMA 2020 and has been adopted to facilitate implementation of the guidance. 60 61 An expanded checklist, which comprises an abridged version of the elements presented in the explanation and elaboration paper, with references and some examples removed, is available in the data supplement on bmj.com. Consulting the explanation and elaboration paper is recommended if further clarity or information is required.

Journals and publishers might impose word and section limits, and limits on the number of tables and figures allowed in the main report. In such cases, if the relevant information for some items already appears in a publicly accessible review protocol, referring to the protocol may suffice. Alternatively, placing detailed descriptions of the methods used or additional results (such as for less critical outcomes) in supplementary files is recommended. Ideally, supplementary files should be deposited to a general-purpose or institutional open-access repository that provides free and permanent access to the material (such as Open Science Framework, Dryad, figshare). A reference or link to the additional information should be included in the main report. Finally, although PRISMA 2020 provides a template for where information might be located, the suggested location should not be seen as prescriptive; the guiding principle is to ensure the information is reported.

Use of PRISMA 2020 has the potential to benefit many stakeholders. Complete reporting allows readers to assess the appropriateness of the methods, and therefore the trustworthiness of the findings. Presenting and summarising characteristics of studies contributing to a synthesis allows healthcare providers and policy makers to evaluate the applicability of the findings to their setting. Describing the certainty in the body of evidence for an outcome and the implications of findings should help policy makers, managers, and other decision makers formulate appropriate recommendations for practice or policy. Complete reporting of all PRISMA 2020 items also facilitates replication and review updates, as well as inclusion of systematic reviews in overviews (of systematic reviews) and guidelines, so teams can leverage work that is already done and decrease research waste. 36 62 63

We updated the PRISMA 2009 statement by adapting the EQUATOR Network’s guidance for developing health research reporting guidelines. 64 We evaluated the reporting completeness of published systematic reviews, 17 21 36 37 reviewed the items included in other documents providing guidance for systematic reviews, 38 surveyed systematic review methodologists and journal editors for their views on how to revise the original PRISMA statement, 35 discussed the findings at an in-person meeting, and prepared this document through an iterative process. Our recommendations are informed by the reviews and survey conducted before the in-person meeting, theoretical considerations about which items facilitate replication and help users assess the risk of bias and applicability of systematic reviews, and co-authors’ experience with authoring and using systematic reviews.

Various strategies to increase the use of reporting guidelines and improve reporting have been proposed. They include educators introducing reporting guidelines into graduate curricula to promote good reporting habits of early career scientists 65 ; journal editors and regulators endorsing use of reporting guidelines 18 ; peer reviewers evaluating adherence to reporting guidelines 61 66 ; journals requiring authors to indicate where in their manuscript they have adhered to each reporting item 67 ; and authors using online writing tools that prompt complete reporting at the writing stage. 60 Multi-pronged interventions, where more than one of these strategies are combined, may be more effective (such as completion of checklists coupled with editorial checks). 68 However, of 31 interventions proposed to increase adherence to reporting guidelines, the effects of only 11 have been evaluated, mostly in observational studies at high risk of bias due to confounding. 69 It is therefore unclear which strategies should be used. Future research might explore barriers and facilitators to the use of PRISMA 2020 by authors, editors, and peer reviewers, designing interventions that address the identified barriers, and evaluating those interventions using randomised trials. To inform possible revisions to the guideline, it would also be valuable to conduct think-aloud studies 70 to understand how systematic reviewers interpret the items, and reliability studies to identify items where there is varied interpretation of the items.

We encourage readers to submit evidence that informs any of the recommendations in PRISMA 2020 (via the PRISMA statement website: http://www.prisma-statement.org/ ). To enhance accessibility of PRISMA 2020, several translations of the guideline are under way (see available translations at the PRISMA statement website). We encourage journal editors and publishers to raise awareness of PRISMA 2020 (for example, by referring to it in journal “Instructions to authors”), endorsing its use, advising editors and peer reviewers to evaluate submitted systematic reviews against the PRISMA 2020 checklists, and making changes to journal policies to accommodate the new reporting recommendations. We recommend existing PRISMA extensions 47 49 50 51 52 53 71 72 be updated to reflect PRISMA 2020 and advise developers of new PRISMA extensions to use PRISMA 2020 as the foundation document.

We anticipate that the PRISMA 2020 statement will benefit authors, editors, and peer reviewers of systematic reviews, and different users of reviews, including guideline developers, policy makers, healthcare providers, patients, and other stakeholders. Ultimately, we hope that uptake of the guideline will lead to more transparent, complete, and accurate reporting of systematic reviews, thus facilitating evidence based decision making.

Acknowledgments

We dedicate this paper to the late Douglas G Altman and Alessandro Liberati, whose contributions were fundamental to the development and implementation of the original PRISMA statement.

We thank the following contributors who completed the survey to inform discussions at the development meeting: Xavier Armoiry, Edoardo Aromataris, Ana Patricia Ayala, Ethan M Balk, Virginia Barbour, Elaine Beller, Jesse A Berlin, Lisa Bero, Zhao-Xiang Bian, Jean Joel Bigna, Ferrán Catalá-López, Anna Chaimani, Mike Clarke, Tammy Clifford, Ioana A Cristea, Miranda Cumpston, Sofia Dias, Corinna Dressler, Ivan D Florez, Joel J Gagnier, Chantelle Garritty, Long Ge, Davina Ghersi, Sean Grant, Gordon Guyatt, Neal R Haddaway, Julian PT Higgins, Sally Hopewell, Brian Hutton, Jamie J Kirkham, Jos Kleijnen, Julia Koricheva, Joey SW Kwong, Toby J Lasserson, Julia H Littell, Yoon K Loke, Malcolm R Macleod, Chris G Maher, Ana Marušic, Dimitris Mavridis, Jessie McGowan, Matthew DF McInnes, Philippa Middleton, Karel G Moons, Zachary Munn, Jane Noyes, Barbara Nußbaumer-Streit, Donald L Patrick, Tatiana Pereira-Cenci, Ba’ Pham, Bob Phillips, Dawid Pieper, Michelle Pollock, Daniel S Quintana, Drummond Rennie, Melissa L Rethlefsen, Hannah R Rothstein, Maroeska M Rovers, Rebecca Ryan, Georgia Salanti, Ian J Saldanha, Margaret Sampson, Nancy Santesso, Rafael Sarkis-Onofre, Jelena Savović, Christopher H Schmid, Kenneth F Schulz, Guido Schwarzer, Beverley J Shea, Paul G Shekelle, Farhad Shokraneh, Mark Simmonds, Nicole Skoetz, Sharon E Straus, Anneliese Synnot, Emily E Tanner-Smith, Brett D Thombs, Hilary Thomson, Alexander Tsertsvadze, Peter Tugwell, Tari Turner, Lesley Uttley, Jeffrey C Valentine, Matt Vassar, Areti Angeliki Veroniki, Meera Viswanathan, Cole Wayant, Paul Whaley, and Kehu Yang. We thank the following contributors who provided feedback on a preliminary version of the PRISMA 2020 checklist: Jo Abbott, Fionn Büttner, Patricia Correia-Santos, Victoria Freeman, Emily A Hennessy, Rakibul Islam, Amalia (Emily) Karahalios, Kasper Krommes, Andreas Lundh, Dafne Port Nascimento, Davina Robson, Catherine Schenck-Yglesias, Mary M Scott, Sarah Tanveer and Pavel Zhelnov. We thank Abigail H Goben, Melissa L Rethlefsen, Tanja Rombey, Anna Scott, and Farhad Shokraneh for their helpful comments on the preprints of the PRISMA 2020 papers. We thank Edoardo Aromataris, Stephanie Chang, Toby Lasserson and David Schriger for their helpful peer review comments on the PRISMA 2020 papers.

Contributors: JEM and DM are joint senior authors. MJP, JEM, PMB, IB, TCH, CDM, LS, and DM conceived this paper and designed the literature review and survey conducted to inform the guideline content. MJP conducted the literature review, administered the survey and analysed the data for both. MJP prepared all materials for the development meeting. MJP and JEM presented proposals at the development meeting. All authors except for TCH, JMT, EAA, SEB, and LAM attended the development meeting. MJP and JEM took and consolidated notes from the development meeting. MJP and JEM led the drafting and editing of the article. JEM, PMB, IB, TCH, LS, JMT, EAA, SEB, RC, JG, AH, TL, EMW, SM, LAM, LAS, JT, ACT, PW, and DM drafted particular sections of the article. All authors were involved in revising the article critically for important intellectual content. All authors approved the final version of the article. MJP is the guarantor of this work. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.

Funding: There was no direct funding for this research. MJP is supported by an Australian Research Council Discovery Early Career Researcher Award (DE200101618) and was previously supported by an Australian National Health and Medical Research Council (NHMRC) Early Career Fellowship (1088535) during the conduct of this research. JEM is supported by an Australian NHMRC Career Development Fellowship (1143429). TCH is supported by an Australian NHMRC Senior Research Fellowship (1154607). JMT is supported by Evidence Partners Inc. JMG is supported by a Tier 1 Canada Research Chair in Health Knowledge Transfer and Uptake. MML is supported by The Ottawa Hospital Anaesthesia Alternate Funds Association and a Faculty of Medicine Junior Research Chair. TL is supported by funding from the National Eye Institute (UG1EY020522), National Institutes of Health, United States. LAM is supported by a National Institute for Health Research Doctoral Research Fellowship (DRF-2018-11-ST2-048). ACT is supported by a Tier 2 Canada Research Chair in Knowledge Synthesis. DM is supported in part by a University Research Chair, University of Ottawa. The funders had no role in considering the study design or in the collection, analysis, interpretation of data, writing of the report, or decision to submit the article for publication.

Competing interests: All authors have completed the ICMJE uniform disclosure form at http://www.icmje.org/conflicts-of-interest/ and declare: EL is head of research for the BMJ ; MJP is an editorial board member for PLOS Medicine ; ACT is an associate editor and MJP, TL, EMW, and DM are editorial board members for the Journal of Clinical Epidemiology ; DM and LAS were editors in chief, LS, JMT, and ACT are associate editors, and JG is an editorial board member for Systematic Reviews . None of these authors were involved in the peer review process or decision to publish. TCH has received personal fees from Elsevier outside the submitted work. EMW has received personal fees from the American Journal for Public Health , for which he is the editor for systematic reviews. VW is editor in chief of the Campbell Collaboration, which produces systematic reviews, and co-convenor of the Campbell and Cochrane equity methods group. DM is chair of the EQUATOR Network, IB is adjunct director of the French EQUATOR Centre and TCH is co-director of the Australasian EQUATOR Centre, which advocates for the use of reporting guidelines to improve the quality of reporting in research articles. JMT received salary from Evidence Partners, creator of DistillerSR software for systematic reviews; Evidence Partners was not involved in the design or outcomes of the statement, and the views expressed solely represent those of the author.

Provenance and peer review: Not commissioned; externally peer reviewed.

Patient and public involvement: Patients and the public were not involved in this methodological research. We plan to disseminate the research widely, including to community participants in evidence synthesis organisations.

This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY 4.0) license, which permits others to distribute, remix, adapt and build upon this work, for commercial use, provided the original work is properly cited. See: http://creativecommons.org/licenses/by/4.0/ .

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Methodology

• Systematic Review | Definition, Example, & Guide

Systematic Review | Definition, Example & Guide

Published on June 15, 2022 by Shaun Turney . Revised on November 20, 2023.

A systematic review is a type of review that uses repeatable methods to find, select, and synthesize all available evidence. It answers a clearly formulated research question and explicitly states the methods used to arrive at the answer.

They answered the question “What is the effectiveness of probiotics in reducing eczema symptoms and improving quality of life in patients with eczema?”

In this context, a probiotic is a health product that contains live microorganisms and is taken by mouth. Eczema is a common skin condition that causes red, itchy skin.

Table of contents

What is a systematic review, systematic review vs. meta-analysis, systematic review vs. literature review, systematic review vs. scoping review, when to conduct a systematic review, pros and cons of systematic reviews, step-by-step example of a systematic review, other interesting articles, frequently asked questions about systematic reviews.

A review is an overview of the research that’s already been completed on a topic.

What makes a systematic review different from other types of reviews is that the research methods are designed to reduce bias . The methods are repeatable, and the approach is formal and systematic:

• Formulate a research question
• Develop a protocol
• Search for all relevant studies
• Apply the selection criteria
• Extract the data
• Synthesize the data
• Write and publish a report

Although multiple sets of guidelines exist, the Cochrane Handbook for Systematic Reviews is among the most widely used. It provides detailed guidelines on how to complete each step of the systematic review process.

Systematic reviews are most commonly used in medical and public health research, but they can also be found in other disciplines.

Systematic reviews typically answer their research question by synthesizing all available evidence and evaluating the quality of the evidence. Synthesizing means bringing together different information to tell a single, cohesive story. The synthesis can be narrative ( qualitative ), quantitative , or both.

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Systematic reviews often quantitatively synthesize the evidence using a meta-analysis . A meta-analysis is a statistical analysis, not a type of review.

A meta-analysis is a technique to synthesize results from multiple studies. It’s a statistical analysis that combines the results of two or more studies, usually to estimate an effect size .

A literature review is a type of review that uses a less systematic and formal approach than a systematic review. Typically, an expert in a topic will qualitatively summarize and evaluate previous work, without using a formal, explicit method.

Although literature reviews are often less time-consuming and can be insightful or helpful, they have a higher risk of bias and are less transparent than systematic reviews.

Similar to a systematic review, a scoping review is a type of review that tries to minimize bias by using transparent and repeatable methods.

However, a scoping review isn’t a type of systematic review. The most important difference is the goal: rather than answering a specific question, a scoping review explores a topic. The researcher tries to identify the main concepts, theories, and evidence, as well as gaps in the current research.

Sometimes scoping reviews are an exploratory preparation step for a systematic review, and sometimes they are a standalone project.

A systematic review is a good choice of review if you want to answer a question about the effectiveness of an intervention , such as a medical treatment.

To conduct a systematic review, you’ll need the following:

• A precise question , usually about the effectiveness of an intervention. The question needs to be about a topic that’s previously been studied by multiple researchers. If there’s no previous research, there’s nothing to review.
• If you’re doing a systematic review on your own (e.g., for a research paper or thesis ), you should take appropriate measures to ensure the validity and reliability of your research.
• Access to databases and journal archives. Often, your educational institution provides you with access.
• Time. A professional systematic review is a time-consuming process: it will take the lead author about six months of full-time work. If you’re a student, you should narrow the scope of your systematic review and stick to a tight schedule.
• Bibliographic, word-processing, spreadsheet, and statistical software . For example, you could use EndNote, Microsoft Word, Excel, and SPSS.

A systematic review has many pros .

• They minimize research bias by considering all available evidence and evaluating each study for bias.
• Their methods are transparent , so they can be scrutinized by others.
• They’re thorough : they summarize all available evidence.
• They can be replicated and updated by others.

Systematic reviews also have a few cons .

• They’re time-consuming .
• They’re narrow in scope : they only answer the precise research question.

The 7 steps for conducting a systematic review are explained with an example.

Step 1: Formulate a research question

Formulating the research question is probably the most important step of a systematic review. A clear research question will:

• Allow you to more effectively communicate your research to other researchers and practitioners
• Guide your decisions as you plan and conduct your systematic review

A good research question for a systematic review has four components, which you can remember with the acronym PICO :

• Population(s) or problem(s)
• Intervention(s)
• Comparison(s)

You can rearrange these four components to write your research question:

• What is the effectiveness of I versus C for O in P ?

Sometimes, you may want to include a fifth component, the type of study design . In this case, the acronym is PICOT .

• Type of study design(s)
• The population of patients with eczema
• The intervention of probiotics
• In comparison to no treatment, placebo , or non-probiotic treatment
• The outcome of changes in participant-, parent-, and doctor-rated symptoms of eczema and quality of life
• Randomized control trials, a type of study design

Their research question was:

• What is the effectiveness of probiotics versus no treatment, a placebo, or a non-probiotic treatment for reducing eczema symptoms and improving quality of life in patients with eczema?

Step 2: Develop a protocol

A protocol is a document that contains your research plan for the systematic review. This is an important step because having a plan allows you to work more efficiently and reduces bias.

Your protocol should include the following components:

• Background information : Provide the context of the research question, including why it’s important.
• Research objective (s) : Rephrase your research question as an objective.
• Selection criteria: State how you’ll decide which studies to include or exclude from your review.
• Search strategy: Discuss your plan for finding studies.
• Analysis: Explain what information you’ll collect from the studies and how you’ll synthesize the data.

If you’re a professional seeking to publish your review, it’s a good idea to bring together an advisory committee . This is a group of about six people who have experience in the topic you’re researching. They can help you make decisions about your protocol.

It’s highly recommended to register your protocol. Registering your protocol means submitting it to a database such as PROSPERO or ClinicalTrials.gov .

Step 3: Search for all relevant studies

Searching for relevant studies is the most time-consuming step of a systematic review.

To reduce bias, it’s important to search for relevant studies very thoroughly. Your strategy will depend on your field and your research question, but sources generally fall into these four categories:

• Databases: Search multiple databases of peer-reviewed literature, such as PubMed or Scopus . Think carefully about how to phrase your search terms and include multiple synonyms of each word. Use Boolean operators if relevant.
• Handsearching: In addition to searching the primary sources using databases, you’ll also need to search manually. One strategy is to scan relevant journals or conference proceedings. Another strategy is to scan the reference lists of relevant studies.
• Gray literature: Gray literature includes documents produced by governments, universities, and other institutions that aren’t published by traditional publishers. Graduate student theses are an important type of gray literature, which you can search using the Networked Digital Library of Theses and Dissertations (NDLTD) . In medicine, clinical trial registries are another important type of gray literature.
• Experts: Contact experts in the field to ask if they have unpublished studies that should be included in your review.

At this stage of your review, you won’t read the articles yet. Simply save any potentially relevant citations using bibliographic software, such as Scribbr’s APA or MLA Generator .

• Databases: EMBASE, PsycINFO, AMED, LILACS, and ISI Web of Science
• Handsearch: Conference proceedings and reference lists of articles
• Gray literature: The Cochrane Library, the metaRegister of Controlled Trials, and the Ongoing Skin Trials Register
• Experts: Authors of unpublished registered trials, pharmaceutical companies, and manufacturers of probiotics

Step 4: Apply the selection criteria

Applying the selection criteria is a three-person job. Two of you will independently read the studies and decide which to include in your review based on the selection criteria you established in your protocol . The third person’s job is to break any ties.

To increase inter-rater reliability , ensure that everyone thoroughly understands the selection criteria before you begin.

If you’re writing a systematic review as a student for an assignment, you might not have a team. In this case, you’ll have to apply the selection criteria on your own; you can mention this as a limitation in your paper’s discussion.

You should apply the selection criteria in two phases:

• Based on the titles and abstracts : Decide whether each article potentially meets the selection criteria based on the information provided in the abstracts.
• Based on the full texts: Download the articles that weren’t excluded during the first phase. If an article isn’t available online or through your library, you may need to contact the authors to ask for a copy. Read the articles and decide which articles meet the selection criteria.

It’s very important to keep a meticulous record of why you included or excluded each article. When the selection process is complete, you can summarize what you did using a PRISMA flow diagram .

Next, Boyle and colleagues found the full texts for each of the remaining studies. Boyle and Tang read through the articles to decide if any more studies needed to be excluded based on the selection criteria.

When Boyle and Tang disagreed about whether a study should be excluded, they discussed it with Varigos until the three researchers came to an agreement.

Step 5: Extract the data

Extracting the data means collecting information from the selected studies in a systematic way. There are two types of information you need to collect from each study:

• Information about the study’s methods and results . The exact information will depend on your research question, but it might include the year, study design , sample size, context, research findings , and conclusions. If any data are missing, you’ll need to contact the study’s authors.
• Your judgment of the quality of the evidence, including risk of bias .

You should collect this information using forms. You can find sample forms in The Registry of Methods and Tools for Evidence-Informed Decision Making and the Grading of Recommendations, Assessment, Development and Evaluations Working Group .

Extracting the data is also a three-person job. Two people should do this step independently, and the third person will resolve any disagreements.

They also collected data about possible sources of bias, such as how the study participants were randomized into the control and treatment groups.

Step 6: Synthesize the data

Synthesizing the data means bringing together the information you collected into a single, cohesive story. There are two main approaches to synthesizing the data:

• Narrative ( qualitative ): Summarize the information in words. You’ll need to discuss the studies and assess their overall quality.
• Quantitative : Use statistical methods to summarize and compare data from different studies. The most common quantitative approach is a meta-analysis , which allows you to combine results from multiple studies into a summary result.

Generally, you should use both approaches together whenever possible. If you don’t have enough data, or the data from different studies aren’t comparable, then you can take just a narrative approach. However, you should justify why a quantitative approach wasn’t possible.

Boyle and colleagues also divided the studies into subgroups, such as studies about babies, children, and adults, and analyzed the effect sizes within each group.

Step 7: Write and publish a report

The purpose of writing a systematic review article is to share the answer to your research question and explain how you arrived at this answer.

Your article should include the following sections:

• Abstract : A summary of the review
• Introduction : Including the rationale and objectives
• Methods : Including the selection criteria, search method, data extraction method, and synthesis method
• Results : Including results of the search and selection process, study characteristics, risk of bias in the studies, and synthesis results
• Discussion : Including interpretation of the results and limitations of the review
• Conclusion : The answer to your research question and implications for practice, policy, or research

To verify that your report includes everything it needs, you can use the PRISMA checklist .

Once your report is written, you can publish it in a systematic review database, such as the Cochrane Database of Systematic Reviews , and/or in a peer-reviewed journal.

In their report, Boyle and colleagues concluded that probiotics cannot be recommended for reducing eczema symptoms or improving quality of life in patients with eczema. Note Generative AI tools like ChatGPT can be useful at various stages of the writing and research process and can help you to write your systematic review. However, we strongly advise against trying to pass AI-generated text off as your own work.

If you want to know more about statistics , methodology , or research bias , make sure to check out some of our other articles with explanations and examples.

• Student’s  t -distribution
• Normal distribution
• Null and Alternative Hypotheses
• Chi square tests
• Confidence interval
• Quartiles & Quantiles
• Cluster sampling
• Stratified sampling
• Data cleansing
• Reproducibility vs Replicability
• Peer review
• Prospective cohort study

Research bias

• Implicit bias
• Cognitive bias
• Placebo effect
• Hawthorne effect
• Hindsight bias
• Affect heuristic
• Social desirability bias

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

A systematic review is secondary research because it uses existing research. You don’t collect new data yourself.

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Systematic Reviews

Aims and scope.

Systematic Reviews encompasses all aspects of the design, conduct and reporting of systematic reviews. The journal publishes high quality systematic review products including systematic review protocols, systematic reviews related to a very broad definition of human health, rapid reviews, updates of already completed systematic reviews, and methods research related to the science of systematic reviews, such as decision modelling. At this time Systematic Reviews does not accept reviews of in vitro studies.  The journal also aims to ensure that the results of all well-conducted systematic reviews are published, regardless of their outcome.

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Older adults’ experiences during the COVID-19 pandemic: a qualitative systematic literature review

• Elfriede Derrer-Merk   ORCID: orcid.org/0000-0001-7241-0808 1 ,
• Maria-Fernanda Reyes-Rodriguez   ORCID: orcid.org/0000-0002-2645-5092 2 ,
• Laura K. Soulsby   ORCID: orcid.org/0000-0001-9071-8654 1 ,
• Louise Roper   ORCID: orcid.org/0000-0002-2918-7628 3 &
• Kate M. Bennett   ORCID: orcid.org/0000-0003-3164-6894 1  

BMC Geriatrics volume  23 , Article number:  580 ( 2023 ) Cite this article

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Relatively little is known about the lived experiences of older adults during the COVID-19 pandemic. We systematically review the international literature to understand the lived experiences of older adult’s experiences during the pandemic.

Design and methodology

This study uses a meta-ethnographical approach to investigate the included studies. The analyses were undertaken with constructivist grounded theory.

Thirty-two studies met the inclusion criteria and only five papers were of low quality. Most, but not all studies, were from the global north. We identified three themes: desired and challenged wellbeing; coping and adaptation; and discrimination and intersectionality.

Overall, the studies’ findings were varied and reflected different times during the pandemic. Studies reported the impact of mass media messaging and its mostly negative impact on older adults. Many studies highlighted the impact of the COVID-19 pandemic on participants' social connectivity and well-being including missing the proximity of loved ones and in consequence experienced an increase in anxiety, feeling of depression, or loneliness. However, many studies reported how participants adapted to the change of lifestyle including new ways of communication, and social distancing. Some studies focused on discrimination and the experiences of sexual and gender minority and ethnic minority participants. Studies found that the pandemic impacted the participants’ well-being including suicidal risk behaviour, friendship loss, and increased mental health issues.

The COVID-19 pandemic disrupted and impacted older adults’ well-being worldwide. Despite the cultural and socio-economic differences many commonalities were found. Studies described the impact of mass media reporting, social connectivity, impact of confinement on well-being, coping, and on discrimination. The authors suggest that these findings need to be acknowledged for future pandemic strategies. Additionally, policy-making processes need to include older adults to address their needs. PROSPERO record [CRD42022331714], (Derrer-Merk et al., Older adults’ lived experiences during the COVID-19 pandemic: a systematic review, 2022).

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Introduction

In March 2020 the World Health Organisation declared a pandemic caused by the virus SARS-CoV2 (COVID-19) [ 1 ]. At this time 118,000 cases in 114 countries were identified and 4,291 people had already lost their lives [ 2 ]. By July 2022, there were over 5.7 million active cases and over 6.4 million deaths [ 2 ]. Despite the effort to combat and eliminate the virus globally, new variants of the virus are still a concern. At the start of the pandemic, little was known about who would be most at risk, but emerging data suggested that both people with underlying health conditions and older people had a higher risk of becoming seriously ill [ 3 ]. Thus, countries worldwide imposed health and safety measures aimed at reducing viral transmission and protecting people at higher risk of contracting the virus [ 4 ]. These measures included: national lockdowns with different lengths and frequencies; targeted shopping times for older people; hygiene procedures (wearing masks, washing hands regularly, disinfecting hands); restricting or prohibiting social gatherings; working from home, school closure, and home-schooling.

Research suggests that lockdowns and protective measures impacted on people’s lives, and had a particular impact on older people. They were at higher risk from COVID-19, with greater disease severity and higher mortality compared to younger people [ 5 ]. Older adults were identified as at higher risk as they are more likely to have pre-existing conditions including heart disease, diabetes, and severe respiratory conditions [ 5 ]. Additionally, recent research highlights that COVID-19 and its safety measures led to increased mental health problems, including increased feelings of depression, anxiety, social isolation, and loneliness, potentially cognitive decline [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. Other studies reported the consequences of only age-based protective health measures including self-isolation for people older people (e.g. feeling old, losing out the time with family) [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ].

Over the past decade, the World Health Organisation (WHO) has recognised the importance of risk communication within public health emergency preparedness and response, especially in the context of epidemics and pandemics. Risk communication is defined as “the real-time exchange of information, advice and opinions between experts or officials, and people who face a threat (hazard) to their survival, health or economic or social well-being” ([ 31 ], p5). This includes reporting the risk and health protection measurements through media and governmental bodies. Constructing awareness and building trust in society are essential components of risk communication [ 32 ]. In the context of the pandemic, the WHO noted that individual risk perception helped to prompt problem-solving activities (such as wearing face masks, social distancing, and self-isolation). However, the prolonged perception of pandemic-related uncertainty and risk could also lead to heightened feelings of distress and anxiety [ 31 , 33 ], see also [ 34 , 35 , 36 , 37 ].

This new and unprecedented disease provided the ground for researchers worldwide to investigate the COVID-19 pandemic. To date (August 2022), approximately 8072 studies have been recorded on the U.S. National Library of Medicine ClinicalTrials.gov [ 38 ] and 12002 systematic reviews have been registered at PROSPERO, concerning COVID-19. However, to our knowledge, there is little known about qualitative research as a response to the COVID-19 pandemic and how it impacted older adults’ well-being [ 39 ]. In particular, little is known about how older people experienced the pandemic. Thus, our research question considers: How did older adults experience the COVID-19 pandemic worldwide?

We use a qualitative evidence synthesis (QES) recommended by Cochrane Qualitative and Implementation Methods Group to identify peer-reviewed articles [ 40 ]. This provides an overview of existing research, identifies potential research gaps, and develops new cumulative knowledge concerning the COVID-19 pandemic and older adults’ experiences. QES is a valuable method for its potential to contribute to research and policy [ 41 ]. Flemming and Noyes [ 40 ] argue that the evidence synthesis from qualitative research provides a richer interpretation compared to single primary research. They identified an increasing demand for qualitative evidence synthesis from a wide range of “health and social professionals, policymakers, guideline developers and educationalists” (p.1).

Methodology

A systematic literature review requires a specific approach compared to other reviews. Although there is no consensus on how it is conducted, recent systematic literature reviews have agreed the following reporting criteria are addressed [ 42 , 43 ]: (a) a research question; (b) reporting database, and search strategy; (c) inclusion and exclusion criteria; (d) reporting selection methods; (e) critically appraisal tools; (f) data analysis and synthesis. We applied these criteria in our study and began by registering the research protocol with Prospero [ 44 ].

The study is registered at Prospero [ 44 ]. This systematic literature review incorporates qualitative studies concerning older adults’ experiences during the COVID-19 pandemic.

Search strategy

The primary qualitative articles were identified via a systematic search as per the qualitative-specific SPIDER approach [ 45 ]. The SPIDER tool is designed to structure qualitative research questions, focusing less on interventions and more on study design, and ‘samples’ rather than populations, encompassing:

S-Sample. This includes all articles concerning older adults aged 60 +  [ 1 ].

P-Phenomena of Interest. How did older adults experience the COVID-19 pandemic?

D-Design. We aim to investigate qualitative studies concerning the experiences of older adults during the COVID-19 pandemic.

E-Evaluation. The evaluation of studies will be evaluated with the amended Critical Appraisal Skills Programme CASP [ 46 ].

R-Research type Qualitative

Information source

The following databases were searched: PsychInfo, Medline, CINAHL, Web of Science, Annual Review, Annual Review of Gerontology, and Geriatrics. A hand search was conducted on Google Scholar and additional searches examined the reference lists of the included papers. The keyword search included the following terms: (older adults or elderly) AND (COVID-19 or SARS or pandemic) AND (experiences); (older adults) AND (experience) AND (covid-19) OR (coronavirus); (older adults) AND (experience) AND (covid-19 OR coronavirus) AND (Qualitative). Additional hand search terms included e.g. senior, senior citizen, or old age.

Inclusion and exclusion criteria

Articles were included when they met the following criteria: primary research using qualitative methods related to the lived experience of older adults aged 60 + (i.e. the experiences of individuals during the COVID-19 pandemic); peer-reviewed journal articles published in English; related to the COVID-19 pandemic; empirical research; published from 2020 till August 2022.

Articles were excluded when: papers discussed health professionals’ experiences; diagnostics; medical studies; interventions; day-care; home care; or carers; experiences with dementia; studies including hospitals; quantitative studies; mixed-method studies; single-case studies; people under the age of 60; grey literature; scoping reviews, and systematic reviews. We excluded clinical/care-related studies as we wanted to explore the everyday experiences of people aged 60 + . Mixed-method studies were excluded as we were interested in what was represented in solely qualitative studies. However, we acknowledge, that mixed-method studies are valuable for future systematic reviews.

Meta-ethnography

The qualitative synthesis was undertaken by using meta-ethnography. The authors have chosen meta-ethnography over other methodologies as it is an inductive and interpretive synthesis analysis and is uniquely “suited to developing new conceptual models and theories” ([ 47 ], p 2), see also [ 48 ]. Therefore, it combines well with constructivist grounded theory methodology. Meta-ethnography also examines and identifies areas of disagreements between studies [ 48 ].

This is of particular interest as the lived experiences of older adults during the COVID-19 pandemic were likely to be diverse. The method enables the researcher to synthesise the findings (e.g. themes, concepts) from primary studies, acknowledging primary data (quotes) by “using a unique translation synthesis method to transcend the findings of individual study accounts and create higher order” constructs ([ 47 ], p. 2). The following seven steps were applied:

Getting started (identify area of interest). We were interested in the lived experiences of older adults worldwide.

Deciding what was relevant to the initial interest (defining the focus, locating relevant studies, decision to include studies, quality appraisal). We decided on the inclusion and exclusion criteria and an appropriate quality appraisal.

Reading the studies. We used the screening process described below (title, abstract, full text)

Determining how the studies were related (extracting first-order constructs- participants’ quotes and second-order construct- primary author interpretation, clustering the themes from the studies into new categories (Table 3 ).

Translating the studies into one another (comparing and contrasting the studies, checking commonalities or differences of each article) to organise and develop higher-order constructs by using constant comparison (Table 3 ). Translating is the process of finding commonalities between studies [ 48 ].

Synthesising the translation (reciprocal and refutational synthesis, a lines of argument synthesis (interpretation of the relationship between the themes- leads to key themes and constructs of higher order; creating new meaning, Tables 2 , 3 ),

Expressing the synthesis (writing up the findings) [ 47 , 48 ].

Screening and Study Selection

A 4-stage screening protocol was followed (Fig.  1 Prisma). First, all selected studies were screened for duplicates, which were deleted. Second, all remaining studies were screened for eligibility, and non-relevant studies were excluded at the preliminary stage. These screening steps were as follows: 1. title screening; 2. abstract screening, by the first and senior authors independently; and 3. full-text screening which was undertaken for almost all papers by the first author. However, 2 papers [ 9 , 23 ] were assessed independently by LS, LR, and LMM to avoid a conflict of interest. The other co-authors also screened independently a portion of the papers each, to ensure that each paper had two independent screens to determine inclusion in the review [ 49 ]. This avoided bias and confirmed the eligibility of the included papers (Fig.  1 ). Endnote reference management was used to store the articles and aid the screening process.

Prisma flow diagram adapted from Page et al. [ 50 ]. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ, 372, n71. https://doi.org/10.1136/bmj.n71 )

Data extraction

After title and abstract screening, 39 papers were selected for reading the full article. 7 papers were excluded after the full-text assessment (1 study was conducted in 2017, but published in 2021; 2 papers were not fully available in English, 2 papers did not address the research question, 1 article was based on a conference abstract only, 1 article had only one participant age 65 +).

The full-text screening included 32 studies. All the included studies, alongside the CASP template, data extraction table, the draft of this article, and translation for synthesising the findings [ 47 , 48 ] were available and accessible on google drive for all co-authors. All authors discussed the findings in regular meetings.

Quality appraisal

A critical appraisal tool assesses a study for its trustworthiness, methodological rigor, and biases and ensures “transparency in the assessment of primary research” ([ 51 ], p. 5); see also [ 48 , 49 , 50 , 51 , 52 , 53 ]. There is currently no gold standard for assessing primary qualitative studies, but different authors agreed that the amended CASPS checklist was appropriate to assess qualitative studies [ 46 , 54 ]. Thus, we use the amended CASP appraisal tool [ 42 ]. The amended CASP appraisal tool aims to improve qualitative evidence synthesis by assessing ontology and epistemology (Table 1 CASP appraisal tool).

A numerical score was assigned to each question to indicate whether the criteria had been met (= 2), partially met (= 1), or not met (= 0) [ 54 ]; see also [ 55 ]. The score 16 – 22 are considered to be moderate and high-quality studies. The studies scored 15 and below were identified as low-quality papers. Although we focus on higher-quality papers, we did not exclude papers to avoid the exclusion of insightful and meaningful data [ 42 , 48 , 52 , 53 , 54 , 55 , 56 , 57 ]. The quality of the paper was considered in developing the evidence synthesis.

We followed the appraisal questions applied for each included study and answered the criteria either ‘Yes’, ‘Cannot tell’, or ‘No’. (Table 1 CASP appraisal criteria). The tenth question asking the value of the article was answered with ‘high’ of importance, ‘middle’, or low of importance. The new eleventh question in the CASP tool concerning ontology and epistemology was answered with yes, no, or partly (Table 1 ).

Data synthesis

The data synthesis followed the seven steps of Meta-Ethnography developed by Noblit & Hare [ 58 ], starting the data synthesis at step 3, described in detail by [ 47 ]. This encompasses: reading the studies; determining how the studies are related; translating the studies into one another; synthesis the translations; and expressing synthesis. This review provides a synthesis of the findings from studies related to the experiences of older adults during the COVID-19 pandemic. The qualitative analyses are based on constructivist grounded theory [ 59 ] to identify the experiences of older adults during the COVID-19 pandemic (non-clinical) populations. The analysis is inductive and iterative, uses constant comparison, and aims to develop a theory. The qualitative synthesis encompasses all text labelled as ‘results’ or ‘findings’ and uses this as raw data. The raw data includes participant’s quotes; thus, the synthesis is grounded in the participant's experience [ 47 , 48 , 60 , 61 ]. The initial coding was undertaken for each eligible article line by line. Please see Table 2 Themes per author and country. Focused coding was applied using constant comparison, which is a widely used approach in grounded theory [ 61 ]. In particular, common and recurring as well as contradicting concepts within the studies were identified, clustered into categories, and overarching higher order constructs were developed [ 47 , 48 , 60 ] (Tables 2 , 3 , 4 ).

We identified twenty-seven out of thirty-two studies as moderate-high quality; they met most of the criteria (scoring 16/22 or above on the CASP; [ 54 ]. Only five papers were identified as low qualitative papers scoring 15 and below [ 71 , 73 , 74 , 86 , 91 ]. Please see the scores provided for each paper in Table 4 . The low-quality papers did not provide sufficient details regarding the researcher’s relationship with the participants, sampling and recruitment, data collection, rigor in the analysis, or epistemological or ontological reasoning. For example, Yildirim [ 91 ] used verbatim notes as data without recording or transcribing them. This article described the analytical process briefly but was missing a discussion of the applied reflexivity of using verbatim notes and its limitations [ 92 ].

This systematic review found that many studies did not mention the relationship between the authors and the participant. The CASP critical appraisal tool asks: Has the relationship between the researcher and participants been adequately considered? (reflecting on own role, potential bias). Many studies reported that the recruitment was drawn from larger studies and that the qualitative study was a sub-study. Others reported that participants contacted the researcher after advertising the study. One study Goins et al., [ 72 ] reported that students recruited family members, but did not discuss how this potential bias impacted the results.

Our review brings new insights into older adults’ experiences during the pandemic worldwide. The studies were conducted on almost all continents. The majority of the articles were written in Europe followed by North America and Canada (4: USA; 3: Canada, UK; 2: Brazil, India, Netherlands, Sweden, Turkey 2; 1: Austria, China, Finland, India/Iran, Mauritius, New Zealand, Serbia, Spain, Switzerland, Uganda, UK/Ireland, UK/Colombia) (see Fig.  2 ). Note, as the review focuses on English language publications, we are unable to comment on qualitative research conducted in other languages see [ 72 ].

Numbers of publications by country

The characteristics of the included studies and the presence of analytical themes can be found in Table 4 . We used the following characteristics: Author and year of publication, research aims, the country conducted, Participant’s age, number of participants, analytical methodology, CASP score, and themes.

We identified three themes: desired and challenged wellbeing; coping and adaptation; discrimination and intersectionality. We will discuss the themes in turn.

Desired and challenged wellbeing

Most of the studies reported the impact of the COVID-19 pandemic on the well-being of older adults. Factors which influenced wellbeing included: risk communication and risk perception; social connectivity; confinement (at home); and means of coping and adapting. In this context, well-being refers to the evidence reported about participants' physical and mental health, and social connectivity.

Risk perception and risk communication

Politicians and media transmitted messages about the response to the pandemic to the public worldwide. These included mortality and morbidity reports, and details of health and safety regulations like social distancing, shielding- self-isolation, or wearing masks [ 34 , 35 , 36 , 37 ]. As this risk communication is crucial to combat the spread of the virus, it is also important to understand how people perceived the reporting during the pandemic.

Seven studies reported on how the mass media impacted participants' well-being [ 23 , 67 , 68 , 70 , 72 , 81 , 85 ]. Sangrar et al. [ 68 ] investigated how older adults responded to COVID-19 messaging: “My reaction was to try to make sure that I listen to everything and [I] made sure I was aware of all the suggestions and the precautions that were being expressed by various agencies …”. (p. 4). Other studies reported the negative impact on participants' well-being of constant messaging and as a consequence stopped watching the news to maintain emotional well-being [ 3 , 67 , 68 , 70 , 72 , 81 , 85 ]. Derrer-Merk et al. [ 23 ] reported one participant said that “At first, watching the news every day is depressing and getting more and more depressing by the day, so I’ve had to stop watching it for my own peace of mind” (p. 13). In addition, news reporting impacted participants’ risk perception. For example, “Sometimes we are scared to hear the huge coverage of COVID-19 news, in particular the repeated message ‘older is risky’, although the message is useful.” ([ 81 ], p5).

• Social connectivity

Social connectivity and support from family and community were found in fourteen of the studies as important themes [ 9 , 62 , 66 , 67 , 68 , 75 , 76 , 77 , 78 , 79 , 80 , 83 , 84 , 90 ].

The impact of COVID-19 on social networks highlighted the diverse experiences of participants. Some participants reported that the size of social contact was reduced: “We have been quite isolated during this corona time” ?([ 80 ], p. 3). Whilst other participants reported that the network was stable except that the method of contact was different: “These friends and relatives, they visited and called as often as before, but of course, we needed to use the telephone when it was not possible to meet” ([ 77 ], p. 5). Many participants in this study did not want to expand their social network see also [ 9 , 77 , 78 , 79 ]. Hafford-Letchfield et al. [ 76 ] reported that established social networks and relationships were beneficial for the participants: “Covid has affected our relationship (with partner), we spend some really positive close time together and support each other a lot” (p. 7).

On the other hand, other studies reported decreases of, and gaps in, social connectedness: “I couldn’t do a lot of things that I’ve been doing for years. That was playing competitive badminton three times a week, I couldn’t do that. I couldn’t get up early and go volunteer in Seattle” [ 9 , 67 , 75 ]. A loss of social connection with children and grandchildren was often mentioned: “We cannot see our grandchildren up close and personal because, well because they [the parents] don’t want us, they don’t want to risk our being with the kids … it’s been an emotional loss exacerbated by the COVID thing” ([ 68 ] p.10); see also [ 9 , 67 , 78 ]. On the contrary, Chemen & Gopalla [ 66 ] note that those older adults who were living with other family members reported that they were more valued: “Last night my daughter-in-law thanked me for helping with my granddaughter” (p.4).

Despite reports of social disconnectedness, some studies highlighted the importance of support from family members and how support changed during the COVID-19 pandemic [ 9 , 62 , 81 , 83 , 90 ]. Yang et al. [ 90 ] argued that social support was essential during the Lockdown in China: “N6 said: ‘I asked my son-in-law to take me to the hospital” (p. 4810). Mahapatra et al. [ 81 ] found, in an Indian study, that the complex interplay of support on different levels (individual, family, and community) helped participants to adapt to the new situation. For example, this participant reported that: “The local police are very helpful. When I rang them for something and asked them to find out about it, they responded immediately” (p. 5).

Impact of confinement on well being

Most articles highlighted the impact of confinement on older adults’ well-being [ 9 , 62 , 63 , 65 , 67 , 69 , 70 , 72 , 75 , 77 , 78 , 79 , 81 , 82 , 83 , 85 , 89 , 90 ].

Some studies found that participants maintained emotional well-being during the pandemic and it did not change their lifestyle [ 79 , 80 , 82 , 83 , 89 , 92 ]: “Actually, I used this crisis period to clean my house. Bookcases are completely cleaned and I discarded old books. Well, we have actually been very busy with those kind of jobs. So, we were not bored at all” ([ 79 ], p. 5). In McKinlay et al. [ 82 ]’s study, nearly half of the participants found that having a sense of purpose helped to maintain their well-being: “You have to have a purpose you see. I think mental resilience is all about having a sense of purpose” (p. 6).

However, at the same time, the majority of the articles (12 out of 18) highlighted the negative impact of confinement and social distancing. Participants talked of increased depressive feelings and anxiety. For example, one of Akkus et al.’s [ 62 ] participants said: “... I am depressed; people died. Terrible disease does not give up, it always kills, I am afraid of it …” (p. 549). Similarly, one of Falvo et al.’s [ 67 ] participants remarked: “I am locked inside my house and I am afraid to go out” (p. 7).

Many of the studies reported the negative impact of loneliness as a result of confinement on participants’ well-being including [ 69 , 70 , 72 , 78 , 79 , 90 , 93 ]. Falvo et al. [ 67 ] reported that many participants experienced loneliness: “What sense does it make when you are not even able to see a family member? I mean, it is the saddest thing not to have the comfort of having your family next to you, to be really alone” (p. 8).

Not all studies found a negative impact on loneliness. For example, a “loner advantage” was found by Xie et al. ([ 82 ], p. 386). In this study participants found benefits in already being alone “It’s just a part of who I am, and I think that helps—if you can be alone, it really is an asset when you have to be alone” ([ 82 ], p. 386).

Bundy et al. [ 80 ] investigated loneliness from already lonely older adults and found that many participants did not attribute the loneliness to the pandemic: “It’s not been a whole lot, because I was already sitting around the house a whole lot anyway ( …). It’s basically the same, pretty well … I’d pretty well be like this anyway with COVID or without COVID” (p. 873) (see also [ 83 ]).

A study from Serbia investigated how the curfew was perceived 15 months afterward. Some participants were calm: “I realized that … well … it was simply necessary. For that reason, we accepted it as a measure that is for the common good” ([ 70 ], p.634). Others were shocked: “Above all, it was a huge surprise and sort of a shock, a complete shock because I have never, ever seen it in my life and I felt horrible, because I thought that something even worse is coming, that I even could not fathom” ([ 70 ], p. 634).

The lockdowns brought not only mental health issues to the fore but impacted the physical health of participants. Some reported they were fearful of the COVID-19 pandemic: “... For a little while I was afraid to leave, to go outside. I didn’t know if you got it from the air” ([ 75 ]. p. 6). Another study reported: “It’s been important for me to walk heartily so that I get a bit sweaty and that I breathe properly so that I fill my lungs—so that I can be prepared—and be as strong as possible, in case I should catch that coronavirus” ([ 77 ], p. 9); see also [ 70 , 78 , 82 , 85 ].

Coping and adaptation

Many studies mentioned older adults’ processes of coping and adaptation during the pandemic [ 63 , 64 , 68 , 69 , 72 , 75 , 79 , 81 , 85 , 87 , 88 , 89 , 90 ].

A variety of coping processes were reported including: acceptance; behavioural adaptation; emotional regulation; creating new routines; or using new technology. Kremers et al. [ 79 ] reported: “We are very realistic about the situation and we all have to go through it. Better days will come” (p. e71). Behavioural adaptation was reported: “Because I’m asthmatic, I was wearing the disposable masks, I really had trouble breathing. But I was determined to find a mask I could wear” ([ 68 ], p. 14). New routines with protective hygiene helped some participants at the beginning of the pandemic to cope with the health threat: “I am washing my hands all the time, my hands are raw from washing them all the time, I don't think I need to wash them as much as I do but I do it just in case, I don’t have anybody coming in, so there is nobody contaminating me, but I keep washing” ([ 69 ], p. 4391); see also [ 72 ]. Verhage et al. [ 87 ] reported strategies of coping including self-enhancing comparisons, distraction, and temporary acceptance: “There are so many people in worse circumstances …” (p. e294). Other studies reported how participants used a new technology: “I have recently learned to use WhatsApp, where I can make video phone calls.” ([ 88 ], p. 163); see also [ 89 ].

Discrimination -intersectionality (age and race/gender identity)

Seven studies reported ageism, racism, and gender discrimination experienced by older adults during the pandemic [ 23 , 63 , 67 , 70 , 76 , 84 , 88 ].

Prigent et al. [ 84 ], conducted in a New Zealand study, found that ageism was reciprocal. Younger people spoke against older adults: “why don’t you do everyone a favour and drop dead you f******g b**** it’s all because of ones like you that people are losing jobs” (p. 11). On the other hand, older adults spoke against the younger generation: “Shame to see the much younger generations often flout the rules and generally risk the gains made by the team. Sheer arrogance on their part and no sanctions applied” (p.11). Although one study reported benevolent ageism [ 23 ] most studies found hostile ageism [ 23 , 63 , 67 , 70 , 76 , 84 ]. One study from Canada exploring 15 older adult’s Chinese immigrants’ experiences reported racism as people around them thought they would bring the virus into the country. The negative impact on existing friendships was told by a Chinese man aged 69 “I can tell some people are blatantly despising us. I can feel it. When I talked with my Caucasian friends verbally, they would indirectly blame us for the problem. Eventually, many of our friendships ended because of this issue” ([ 88 ], p161). In addition, this study reported ageism when participants in nursing homes felt neglected by the Canadian government.

Two papers reported experiences of sexual and gender minorities (SGM) (e.g. transgender, queer, lesbian or gay) and found additional burdens during the pandemic [ 63 , 76 ]. People experienced marginalisation, stereotypes, and discrimination, as well as financial crisis: “I have faced this throughout life. Now people look at me in a way as if I am responsible for the virus.” ([ 63 ], p. 6). The consequence of marginalisation and ignorance of people with different gender identities was also noted by Hafford- Letchfield et al. [ 76 ]: “People have been moved out of their accommodation into hotels with people they don't know …. a gay man committed suicide, community members know of several that have attempted suicide. They are feeling pretty marginalised and vulnerable and you see what people are writing on the chat pages” (p.4). The intersection of ageism, racism, and heterosexism and its negative impact on people’s well-being during the pandemic reflects additional burden and stressors for older adults.

This systematic literature review is important as it provides new insights into the lived experiences of older adults during the COVID-19 pandemic, worldwide. Our study highlights that the COVID-19 pandemic brought an increase in English-written qualitative articles to the fore. We found that 32 articles met the inclusion criteria but 5 were low quality. A lack of transparency reduces the trustworthiness of the study for the reader and the scientific community. This is particularly relevant as qualitative research is often criticised for its bias or lack of rigor [ 94 ]. However, their findings are additional evidence for our study.

Our aim was to explore, in a systematic literature review, the lived experiences of older adults during the COVID-19 pandemic worldwide. The evidence highlights the themes of desired and challenged wellbeing, coping and adaptation, and discrimination and intersectionality, on wellbeing.

Perceived risk communication was experienced by many participants as overwhelming and anxiety-provoking. This finding supports Anwar et al.’s [ 37 ] study from the beginning of the pandemic which found, in addition to circulating information, that mass media influenced the public's behaviour and in consequence the spread of disease. The impact can be positive but has also been revealed to be negative as well. They suggest evaluating the role of the mass media in relation to what and how it has been conveyed and perceived. The disrupted social connectivity found in our review supports earlier studies that reported the negative impact of people’s well-being [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ] at the beginning of the pandemic. This finding is important for future health crisis management, as the protective health measures such as confinement or self-isolation had a negative impact on many of the participants’ emotional wellbeing including increased anxiety, feelings of depression, and loneliness during the lockdowns. As a result of our review, future protective health measures should support people’s desire to maintain proximity with their loved ones and friends. However, we want to stress that our findings are mixed.

The ability of older adults to adapt and cope with the health crisis is important: many of the reported studies noted the diverse strategies used by older people to adapt to new circumstances. These included learning new technologies or changing daily routines. Politicians and the media and politicians should recognise both older adults' risk of disease and its consequences, but also their adaptability in the face of fast-changing health measures. This analysis supports studies conducted over the past decades on lifespan development, which found that people learn and adapt livelong to changing circumstances [ 95 , 96 , 97 ].

We found that discrimination against age, race, and gender identity was reported in some studies, in particular exploring participants’ experiences with immigration backgrounds and sexual and gender minorities. These studies highlighted the intersection of age and gender or race and were additional stressors for older adults and support the findings from Ramirez et al. [ 98 ] This review suggests that more research should be conducted to investigate the experiences of minority groups to develop relevant policies for future health crises.

Our review was undertaken two years after the pandemic started. At the cut-off point of our search strategy, no longitudinal studies had been found. However, in December 2022 a longitudinal study conducted in the USA explored older adult’s advice given to others [ 99 ]. They found that fostering and maintaining well-being, having a positive life perspective, and being connected to others were coping strategies during the pandemic [ 100 ]. This study supports the results of the higher order constructs of coping and adaptation in this study. Thus, more longitudinal studies are needed to enhance our understanding of the long-term consequences of the COVID-19 pandemic. The impact of the COVID-19 restrictions on older adults’ lives is evident. We suggest that future strategies and policies, which aim to protect older adults, should not only focus on the physical health threat but also acknowledge older adults' needs including psychological support, social connectedness, and instrumental support. The policies regarding older adult’s protections changed quickly but little is known about older adults’ involvement in decision making [ 100 ]. We suggest including older adults as consultants in policymaking decisions to ensure that their own self-determinism and independence are taken into consideration.

There are some limitations to this study. It did not include the lived experiences of older adults in care facilities or hospitals. The studies were undertaken during the COVID-19 pandemic and therefore data collection was not generally undertaken face-to-face. Thus, many studies included participants who had access to a phone, internet, or email, others could not be contacted. Additionally, we did not include published papers after August 2022. Even after capturing the most commonly used terms and performing additional hand searches, the search terms used might not be comprehensive. The authors found the quality of the papers to be variable, and their credibility was in question. We acknowledge that more qualitative studies might have been published in other languages than English and were not considered in this analysis.

To conclude, this systematic literature review found many similarities in the experiences of older adults during the Covid-19 pandemic despite cultural and socio-economic differences. However, we stress to acknowledge the heterogeneity of the experiences. This study highlights that the interplay of mass media reports of the COVID-19 pandemic and the policies to protect older adults had a direct impact on older adults’ well-being. The intersection of ‘isms’ (ageism, racism, and heterosexism) brought an additional burden for some older adults [ 98 ]. These results and knowledge about the drawbacks of health-protecting measures need to be included in future policies to maintain older adults’ well-being during a health crisis.

Availability of data and materials

The systematic literature review is based on already published articles. And all data analysed during this study are included in this manuscript. No additional data was used.

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Recurrence of post-traumatic stress disorder: systematic review of definitions, prevalence and predictors

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Many people will experience a potentially traumatic event in their lifetime and a minority will go on to develop post-traumatic stress disorder (PTSD). A wealth of literature explores different trajectories of PTSD, focusing mostly on resilient, chronic, recovered and delayed-onset trajectories. Less is known about other potential trajectories such as recurring episodes of PTSD after initial recovery, and to date there has been no estimate of what percentage of those who initially recover from PTSD later go on to experience a recurrence. This systematic review aimed to synthesise existing literature to identify (i) how ‘recurrence’ of PTSD is defined in the literature; (ii) the prevalence of recurrent episodes of PTSD; and (iii) factors associated with recurrence.

A literature search of five electronic databases identified primary, quantitative studies relevant to the research aims. Reference lists of studies meeting pre-defined inclusion criteria were also hand-searched. Relevant data were extracted systematically from the included studies and results are reported narratively.

Searches identified 5,398 studies, and 35 were deemed relevant to the aims of the review. Results showed there is little consensus in the terminology or definitions used to refer to recurrence of PTSD. Because recurrence was defined and measured in different ways across the literature, and prevalence rates were reported in numerous different ways, it was not possible to perform meta-analysis to estimate the prevalence of recurrence. We also found no consistent evidence regarding predictors of PTSD recurrence.

A clear and consistent evidence-based definition of recurrence is urgently needed before the prevalence and predictors of recurrence can be truly understood.

Peer Review reports

Potentially traumatic events are common. Research suggests that over 70% of people will experience a potentially traumatic event (such as witnessing death or serious injury, automobile accident, life-threatening illness or injury, or violent encounter) in their lifetime [ 1 ]. Understandably, these events can be very distressing in the short-term and many people will experience acute post-traumatic symptoms in the immediate aftermath of a traumatic event, including intrusive symptoms (e.g. recurrent unwanted thoughts, nightmares); avoidance symptoms (e.g. emotional numbing, social withdrawing); hyperarousal (e.g. easily startled, feeling ‘on edge’); and physical symptoms (e.g. chest pain, dizziness) [ 2 ]. For the majority, these symptoms will decline naturally without intervention [ 3 ], typically within the first four weeks [ 2 ]. An important minority will find their symptoms persist for longer than a month. Those who continue to experience persistent re-experiencing of the traumatic event; avoidance of stimuli associated with the event; negative alterations in cognitions and mood and alterations in arousal and reactivity, causing clinical distress or functional impairment and not attributable to any other medical condition, are likely to be diagnosed with post-traumatic stress disorder (PTSD) [ 4 ]. Although only a minority of people who experience potentially traumatic events will go on to develop PTSD, it remains one of the most common mental disorders with lifetime prevalence estimated to be between 8% [ 5 ] and 12% [ 6 ]. PTSD is associated with reduced health-related quality of life and physical comorbidities, as well as major socio-economic costs [ 7 ].

The early 2000s saw a shift from studying PTSD itself as an outcome to studying change in symptoms as an outcome [ 8 ], with a wealth of studies using modelling approaches such as latent class growth analysis and latent growth mixture modelling to identify different trajectories of PTSD. Most of this literature identifies four trajectories, two of which are relatively stable trajectories ( chronic , a stable trajectory of post-traumatic stress symptoms, and resilient , a stable trajectory of healthy functioning after an adverse event), and two which display dynamic symptom patterns ( recovered , i.e. decreasing symptoms after an initial diagnosis of PTSD, and delayed-onset , i.e. increasing symptoms not meeting the diagnostic criteria for PTSD until potentially months or even years after traumatic exposure) [ 9 ]. Van de Schoot et al. [ 10 ] suggest that the two trajectories which typically occur less often (chronic and delayed-onset) are at risk of being overlooked by researchers or overwhelmed within the data by the larger trajectories. There may also be other less-researched or less-understood trajectories overlooked to an even greater extent. For example, one previous review [ 11 ] identified limited evidence of another, smaller trajectory referred to as a ‘relapsing’ or ‘recurring’ PTSD trajectory, in which individuals develop PTSD, are free from symptoms for long enough to be considered ‘recovered’, and then experience a recurrence of symptoms.

Recurrence is given relatively little attention in the PTSD literature, perhaps due to limitations of study methodologies and the complexities of studying recurrence. For example, Santiago et al. [ 11 ] note that few studies of PTSD follow participants for more than a year or with more than two assessments. Clearly, it would not be possible for researchers to identify recurrence of PTSD if data is only collected for two time-points: the only possible outcomes would be low symptom levels at each time-point (‘resilience’), high symptoms at each time-point (‘chronic’), or low level of symptoms at one time-point and a high level at the other (either ‘recovery’ or ‘delayed-onset’ depending on time-point at which symptoms were experienced). Additionally, studies which only follow up participants for a year or less are unlikely to clearly identify a recurrent trajectory of PTSD given the time needed to both recover and to experience a recurrent episode. The timing of PTSD assessment is also important: identification of PTSD recurrence relies on studies capturing the presence of symptoms during the recurrence, rather than before it occurs or after recurring symptoms have subsided. Therefore, it is perhaps unsurprising that the majority of the literature does not identify a ‘recurring’ trajectory of PTSD. Even studies which do identify recurrences often group these in with other trajectories: for example, Mota et al. [ 12 ] identified ‘recurrent’ cases of PTSD (individuals who had a lifetime diagnosis in 2002 and another post-2002 diagnosis reported in 2018), but grouped ‘persistent’ and ‘recurrent’ cases of PTSD together. Magruder et al. [ 13 ] identified a group of recurrent cases of PTSD – individuals who had lifetime PTSD pre-1992 but not a current diagnosis in 2002, who then had a diagnosis again in 2021, but these were grouped with ‘chronic’ cases. Karamustafalioglu et al. [ 14 ] simply include an ‘other’ group constituting both recurrent cases (individuals who met the criteria for PTSD diagnosis 1–3 months post-trauma and at the third follow-up 18–20 months post-trauma, but not at the second follow-up 6–10 months post-trauma) and others with delayed-onset PTSD which resolved. Boe et al. [ 15 ] identified a group of individuals with ‘reactivated’ PTSD who reported remission from PTSD in the first five years after the North Sea oil rig disaster of 1980 and a new episode at any point between 1985 and 2007. However, the authors suggest that there are blurred boundaries between delayed-onset and ‘reactivated’ PTSD, going on to include ‘possible delayed cases’ in their analysis of reactivated PTSD.

It is important to note that even the definitions of the more well-established trajectories of PTSD are not without their controversies. For example, Andrews et al. [ 16 ] point out the ambiguity in the criterion for delayed-onset PTSD, questioning whether ‘the onset of symptoms’ refers to any symptoms which might eventually lead to PTSD or only to full-blown PTSD itself. North et al. [ 17 ] comment on the ambiguities involved in the term remission (i.e. whether remission should be symptom-based or threshold-based) as well as the term onset (i.e. whether onset refers to first symptoms or first meeting diagnostic criteria). Definition of recovery also appears to differ from study to study, with some authors considering recovery to be symptom-based (i.e. no symptoms of the disorder remain) and others considering it to be threshold-based (i.e. some symptoms may remain, but they are beneath the diagnostic threshold) [ 18 ].

To date, several systematic reviews have been published which focus solely on only one PTSD trajectory. For example, previous reviews have focused on the delayed-onset trajectory [ 16 , 19 ]; the recovery trajectory [ 20 ]; and the resilient trajectory [ 21 ]. To date there has not been a literature review examining evidence of a recurrent trajectory of PTSD. Berge et al. [ 22 ] aimed to systematically review research on relapse in veterans but found no studies reporting actual rates of relapse or recurrence. Reviews have also explored the risk of relapse of various anxiety disorders, including PTSD, after discontinuation of antidepressants [ 23 ] and after cognitive behavioural therapy [ 24 ]. However, there have been no reviews attempting to quantify the risk of PTSD recurring, establish the predictors of recurrence, or quantify how much each predictive factor contributes to the risk of recurrence. The current review aimed to fill this gap in the literature by synthesising existing published data on how researchers define ‘recurrence’ of PTSD, recurrence rates of PTSD, and predictive factors of recurrence.

Having an appropriate understanding of recurrence is important as the concept needs to be properly understood in order to take steps to mitigate the risks of recurrent PTSD episodes. Mitigating the risk of PTSD recurring could benefit the health and wellbeing of trauma-exposed individuals and could reduce the socio-economic costs to the wider society [ 7 ]. The prevalence of recurrence is of particular importance to occupational medicine: regularly trauma-exposed organisations, for example, are often faced with decisions about when (and if) staff who have had and recovered from PTSD should return to the frontline duties. Understanding the risk of recurrent episodes may therefore have implications for those in charge of making such decisions. The present time is also a particularly relevant time to develop our understanding of recurrence of PTSD, as it is possible that the COVID-19 pandemic could contribute to recurrence. The pandemic has been declared a potential traumatic stressor, with research suggesting that COVID-19 survivors are at elevated risk of experiencing PTSD [ 25 ] and that PTSD symptoms may also develop due to quarantine [ 26 ], concerns about the health of loved ones, or economic loss as a result of the pandemic [ 27 ]. Hori et al. [ 28 ] suggest that the daily television updates regarding COVID-19 could trigger memories of surviving a previous traumatic situation, and exacerbate subthreshold PTSD symptoms. Therefore, experiencing the pandemic could potentially cause a recurrence of symptoms in people who have previously been diagnosed with PTSD.

The aim of this review was to collate literature which provides evidence of the lesser-studied ‘recurrent’ trajectory of PTSD and to identify: (i) the definitions of ‘recurrence’ used throughout the literature; (ii) prevalence of recurrence; and (iii) risk and protective factors for the recurrent trajectory of PTSD.

This review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [ 29 ]. Our population of interest were people who had been diagnosed with, recovered from, and experienced a recurrence of PTSD (as diagnosed by a clinician or validated PTSD assessment tool). For the aim relating to prevalence of recurrent episodes, studies needed to involve a suitable design allowing prevalence to be assessed: for example, studies involving a population of people who had recovered from PTSD, followed over time to show how many had a recurrent episode and how many did not. For the other aims (i.e., definitions of recurrence and factors associated with recurrence), a comparison group was not necessary.

Registering the review

A protocol for the current review was developed and registered with PROSPERO on March 9th 2023 (registration number CRD42023405752). The only deviation from the protocol was the addition of another quality appraisal tool, due to finding a study design (retrospective analysis of existing health data) which we had not anticipated.

Eligibility criteria

To be included in the review, studies needed to (1) be published in peer-reviewed journals, (2) be published in the English language, (3) use quantitative methodology, (4) use a standardised tool to assess PTSD and (5) present data on recurrence rates of PTSD and/or factors associated with PTSD recurrence. There were no limitations relating to publication date or location of the studies. Case studies were excluded but there were no other exclusion criteria relating to population size.

Data searching and screening

A systematic literature search was carried out to examine definitions, prevalence rates and predictors of PTSD recurrence. Four electronic databases (Embase, PsycInfo, Medline and Web of Science) were searched on 24th November 2022, using a combination of search terms relating to PTSD, recurrence, and prevalence/predictors which were combined using Boolean operators. The full list of search terms is presented in Appendix 1 . The US Department of Veterans Affairs National Center for Post-Traumatic Stress Disorder’s PTSDPubs database (formerly PILOTS) was searched separately on the same date using the individual terms ‘recurrence’ and ‘recurrent’ and limited to peer-reviewed articles. Reference lists of articles deemed to meet the inclusion criteria were also hand-searched.

All citations resulting from the literature searches were downloaded to an EndNote library where duplicates were removed. The titles of all citations were then screened for relevance to the review, with any clearly not relevant being excluded. Abstracts were then screened for eligibility and the full texts of all remaining citations after abstract screening were located and read in their entirety to identify studies meeting all inclusion criteria. The literature searches and screening were carried out by the first author. The two authors met regularly throughout the screening process to discuss any uncertainties about inclusion or exclusion until a decision was reached.

Data extraction

The first author carried out data extraction of all citations deemed to meet the inclusion criteria. Data were extracted to a Microsoft Excel spreadsheet with the following headings: authors, year of publication, country, study design, sampling method, inclusion/exclusion criteria, study population size, socio-demographic characteristics of participants, type of trauma exposure, time-points at which PTSD was assessed, tools for assessing PTSD, definitions of recovery and recurrence, whether any PTSD treatment was received, prevalence rates of recurrence, and factors examined as potential predictors of recurrence.

Data synthesis

For the first aim of the review (relating to definitions of recurrence), we designed a table to present data relating to how ‘recurrence’ was understood and defined in each study. The tools used to diagnose and measure PTSD symptoms in the first place are important in understanding how PTSD is defined, so first the assessment tools used in each study were extracted into the table. Given that we wanted to understand the length of time an individual needs to be free of PTSD in order to be considered ‘recovered’, for each study we also included the time-points of PTSD assessment in the table. Next, we included the definitions of recovery and recurrence from each study, explained narratively in the table. We also added information to this table to report whether participants had received PTSD treatment during each study, as some studies focusing on interventions used ‘response to treatment’ in their definitions of recovery. We compared the different definitions used within the studies to establish whether there was consensus within the literature around (i) whether recovery and recurrence are symptom-based or threshold-based and (ii) how long the recovery period between initial diagnosis and recurrent episodes needs to be in order to be considered recurrent rather than chronic PTSD.

The second aim related to prevalence of PTSD recurrence. Due to the various research designs and definitions of ‘recurrence’ in the literature, as well as the different ways in which prevalence was reported, meta-analytic techniques could not be used. Rather, we presented the prevalence data as it was reported in each study. This sometimes meant presenting the prevalence of PTSD recurrence within an entire trauma-exposed population, including those who never experienced PTSD at any time. Other times, this meant presenting the prevalence of PTSD within a population who all had PTSD at one time-point, and other times this meant presenting the prevalence of PTSD within a group who had recovered from PTSD.

Finally, in order to explore factors associated with PTSD recurrence, all variables considered as potential covariates were recorded individually for each study. Each potential predictive factor was descriptively reported in a table, and any found to be significantly associated with experiencing PTSD recurrence were bolded to differentiate between non-significant and significant findings. Factors are also described narratively within the results section. Insights from thematic analysis [ 30 ] were used to group similar data together. For example, data relating to gender or age as predictors of recurrence were coded ‘socio-demographic’ and discussed together within the results.

Quality appraisal

We appraised the quality of studies using National Institutes of Health (NIH) tools: either the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies or the Quality Assessment of Controlled Intervention Studies tool, depending on study design. Concurrent with other reviews [e.g. 31 ] we rated quality as ‘poor’ if studies scored 0–4/14, ‘fair’ if they scored 5–10/14 and ‘good’ if they scored 11–14/14. One study used retrospective analysis of existing health data, and for this study we used the MetaQAT Critical Appraisal Tool [ 32 ]. To keep the ratings consistent with our rating system for the studies appraised by NIH tools, we defined ‘poor’ quality as a score of 0–34%, ‘fair’ quality as a score of 35–72% and ‘good’ quality as a score of 78% or higher.

Literature searches yielded 5,398 citations of which 1,083 were duplicates. After title and abstract screening, 4,210 citations were excluded leaving 105 citations for full-text screening. After reading full texts of the remaining citations, 75 were excluded and an additional five studies were added after hand-searching reference lists. A total of 35 citations were included in the review [ 15 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 ]. Figure  1 illustrates the screening process in a PRISMA flow diagram.

PRISMA flow diagram of screening process

Table  1 provides an overview of key characteristics of all included studies. Studies originated from the United States of America (n = 13), Denmark (n = 5), Israel (n = 4), China (n = 4), Norway (n = 2), the United Kingdom (n = 2), Japan (n = 1), the Netherlands (n = 1), Switzerland (n = 1), and Turkey (n = 1). The remaining study included participants in multiple different countries across Europe and Asia. Study populations ranged from 35 to 7,918 and included military personnel (n = 15), civilian adults (n = 14), children or adolescents (n = 4) or a combination of military and civilian adults (n = 2). Only three studies were rated as ‘good’ quality; the majority were rated ‘fair’.

Definitions of recurrence

Table  2 reports, for each study, the tools used to assess PTSD; time-points at which PTSD was assessed; definitions of recovery and recurrence; and whether the participants received PTSD treatment or not.

Terminology

The first aim of the review was to explore how ‘recurrence’ is defined in the literature. We found no consensus in terms of how this is defined. In fact, the studies used a variety of different terms to describe the emergence of new PTSD episodes after initial ‘recovery’, including ‘recurrence’ [ 33 , 37 , 44 , 47 , 64 , 65 ]; ‘relapse’ [ 35 , 36 , 40 , 49 , 50 , 52 , 53 , 57 ]; ‘reactivation’ [ 15 , 60 , 62 ]; ‘exacerbation/reactivation’ [ 61 ]; ‘relieved-worsening PTSD’ [ 34 , 48 , 51 , 63 ]; ‘response-remit’ trajectory [ 54 ]; ‘fluctuating course’ [ 58 ]; ‘intermittent cases’ [ 43 ]; ‘delayed increase in symptoms’ [ 46 ]; and the ‘relapsing/remitting’ trajectory [ 42 , 55 ]. Many others simply described recurrence as ‘symptom increase’ [ 38 ], ‘initial declines followed by symptom increases’ [ 56 ] or ‘exacerbation of symptoms’ [ 41 , 60 ]. Some studies did not name the trajectory at all; rather, they presented tables or flow charts showing the number of participants with PTSD at each time-point, from which it was possible for us to identify a sub-group of participants who were described as having PTSD at one time-point, not having it at least one follow-up, and then having it again at subsequent time-points [ 39 , 59 ]. Similarly, Hansen et al. [ 45 ] identified and commented on a sub-group of participants who met the criteria for PTSD, did not meet the criteria at a subsequent time-point, and then met the criteria again later, but they did not give this a name.

Criteria for recurrence

Several studies defined recurrence (or equivalent terminology such as relapse) as meeting diagnostic criteria for PTSD at a follow-up time-point after an initial ‘recovery’ period where they did not meet the cut-off for PTSD [ 33 , 35 , 37 , 39 , 43 , 45 , 46 , 58 , 65 ]. Holliday et al. [ 47 ] referred to ‘clinically meaningful change in PTSD symptoms’, which was also assumed to refer to clinical cut-off scores. Markowitz et al. [ 52 ] based the definition of relapse on similarity to baseline scores. Sungur and Kaya [ 64 ] defined recovery and recurrence as being asymptomatic and then symptomatic again, but it is not clear whether this referred to clinical cut-offs. One study defined ‘reactivation’ of PTSD as meeting full diagnostic criteria or being a sub-syndromal case [ 15 ]. Others were more vague and did not mention cut-offs, instead referring to dramatic or steep symptom increases [ 34 , 38 , 56 , 63 ], fluctuating symptoms [ 42 , 55 ], returning to pre-treatment levels of PTSD [ 54 ], symptoms which ‘decreased somewhat and increased drastically’ [ 48 ], symptoms which ‘decreased to a low level and increased again’ [ 49 , 50 ] or ‘steadily worsening’ symptoms [ 36 ]. DenVelde et al. [ 41 ] simply asked participants to self-report whether they had ‘experienced remissions and exacerbations’. Martenyi et al. [ 53 ] had multiple definitions of relapse, including increases in scores on their PTSD measures or ‘the clinical judgement of the investigator’. Others labelled the trajectory but did not specify the parameters of their definitions [ 51 , 60 , 61 , 62 , 66 ]. One study [ 57 ] used ‘being hospitalised’ as a proxy measure of PTSD recurrence, although this way of defining recurrence would obviously not capture individuals who developed recurring symptoms which were not severe enough to warrant hospitalisation; additionally, no criteria for hospitalisation were described. Similarly, Davidson et al. [ 40 ] described ‘relapse’ as PTSD scores reverting back to baseline or worse, or experiencing an ‘untoward clinical event’ including suicidality, hospitalisation, or dropping out of the study due to feeling progress was not being made.

We found little consensus as to how long participants needed to be symptom-free (or have reduced symptoms) in order to be considered ‘recovered’ prior to recurrence. The majority of studies simply based their definitions on the time-points of the study, suggesting that recurrence was identified if participants had PTSD at baseline, did not have PTSD during at least one follow-up, and then had PTSD again at a later follow-up. The time-points of follow-ups ranged from weeks to months to years. Only four studies suggested specific timeframes: three studies claimed that participants needed to be ‘recovered’ for eight weeks in order for later reports of PTSD to count as ‘recurrence’ rather than symptom fluctuation [ 35 , 37 , 66 ] whereas Zanarini et al. [ 65 ] reported that participants needed to be not meeting the PTSD criteria for at least two years in order to be considered ‘recovered’. Similarly, most studies did not clarify a time-scale for how long symptoms needed to be experienced in order to be considered a ‘recurrence’. Most studies again simply based their diagnosis on the scores participants happened to report on the days they were assessed. Few studies specified a time-frame: three [ 35 , 43 , 65 ] suggested a duration of four consecutive weeks of meeting their criteria for PTSD, while Benítez et al. [ 37 ] suggested two weeks of symptoms was sufficient to identify a recurrent episode.

Prevalence of recurrence

The review’s second aim was to explore PTSD recurrence rates. Table  3 presents data on the prevalence of recurrence of PTSD for each study. The second column of Table  3 presents the data that is reported in the original studies. The findings reported in this column are not easily comparable because studies reported recurrence rates in different ways. Some reported the percentage of the entire trauma-exposed sample who experienced PTSD recurrence (column 3 of Table  3 ). Others reported the percentage of those with PTSD who experienced recurrence (column 4 of Table  3 ) and the remaining studies reported the percentage of those who recovered from PTSD who experienced recurrence (column 5 of Table  3 ). Three studies [ 44 , 47 , 57 ] did not report the prevalence of recurrence, but were still included in the review as they included definitions and/or predictors of recurrence. One study [ 60 ] deliberately chose a sample who had all experienced recurrence; therefore, recurrence prevalence data for this study was not recorded in Table  3 as it would, by design, be 100%.

Most studies (19/35) reported the prevalence of recurrence within the entire trauma-exposed population. We would therefore expect prevalence rates to be extremely small, given that the majority of trauma-exposed people will not develop PTSD in the first place [ 3 ], let alone have recurrent episodes. However, in several studies this was not the case. Prevalence of recurrence ranged from 0.2% (for a sub-set of participants who did not directly witness the disaster in question) [ 45 ] to 57% of 63 women newly-diagnosed with ovarian cancer [ 43 ]. The latter study was carried out over 27 weeks and identified ‘intermittent cases’ who had PTSD at one time-point, no PTSD at a later time-point, and then PTSD again later on. We note that 27 weeks is a fairly short period of time for both recovery and recurrence to occur, and it is therefore possible that the data reflects symptom fluctuations rather than true recovery or recurrence. Overall, the mean prevalence of recurrent PTSD in trauma-exposed populations was 13.1%, and the median was 3.8%.

Five studies presented the prevalence of recurrence within populations diagnosed with PTSD. We would expect these prevalence rates to be higher than the prevalence rates of recurrence within full trauma-exposed samples, as they are based on populations who developed PTSD only. The rates were 4.9% [ 39 ], 15.4% [ 66 ], 24.5% [ 36 ], 28% [ 46 ] and 49.6% [ 41 ]. Mean and median prevalence of recurrent PTSD were both 24.5%.

Seven studies presented data on the prevalence of recurrence within sub-sets of study populations who had recovered from PTSD; therefore, the only possible trajectories for these participants would be recurrence or maintenance of recovery. Recurrence rates ranged from 5.8% (for a sub-set of participants treated with fluoxetine) [ 53 ] to 50% (for a sub-group treated with a placebo) [ 40 ]. Mean prevalence of recurrent PTSD was 25.4% and the median was 22.2%.

The three studies rated highest in quality [ 34 , 47 , 55 ] did not report similar findings relating to prevalence. Holliday et al. [ 47 ] did not present prevalence data at all. Andersen et al. [ 34 ] reported that 2% of participants followed the ‘relieved-worsening’ trajectory, whereas Osenbach et al. [ 55 ] reported that 35% of participants followed the ‘relapsing-remitting’ trajectory. Notably, Andersen et al.’s [ 34 ] participants were military personnel, whilst Osenbach et al.’s [ 55 ] participants were civilian trauma survivors. For this reason, we decided to look separately at recurrence rates in military and civilian participants. We also decided to look separately at data on children as children’s experiences during and after potentially traumatic events are likely to be distinct from those of adults [ 67 ]. Table  4 presents the mean and median recurrence rates for different populations.

Prevalence of PTSD recurrence in military populations

Fifteen studies focused on military personnel and veterans, three of which did not provide prevalence data and one of which included only participants with PTSD recurrence. Military studies which presented rates of recurrence in trauma-exposed populations (rather than focusing on people diagnosed with PTSD only) typically found low prevalence of recurrence: seven studies found prevalence rates under 4% [ 34 , 48 , 51 , 54 , 61 , 62 ]. Another study found a prevalence rate of 6% [ 38 ]. The only higher prevalence rates were reported by Solomon & Mikulincer [ 59 ], who reported recurrence rates of 24.4% for those with combat stress reactions (people referred for psychiatric intervention during the war) and 13.2% for participants who participated in combat in the same units but without need for psychiatric intervention during the war. This study assessed participants over twenty years, which may explain its higher prevalence rate than the majority of studies which were completed within two-and-a-half years or less. However, the study period was shorter than the forty-seven years of Solomon et al.’s [ 62 ] study, which reported only a 1.6% rate of recurrence. It is unclear why Solomon and Mikulincer [ 59 ] found much higher rates of recurrence.

Two military studies reported recurrence rates for PTSD-populations. These were 24.5% [ 36 ] and 49.6% [ 41 ]. We note that all of Armenta et al.’s [ 36 ] participants had comorbid depression at baseline. We also note some concerns about the reliability of DenVelde et al.’s study [ 41 ], which was a retrospective study asking participants to give complete life-history data at one time-point only.

One military study reported on the prevalence of recurrence in a sub-group of participants who had recovered. Solomon et al. [ 62 ], who reported a prevalence rate of 1.6% (out of the entire trauma-exposed sample) over the first forty-two years of the study, found in a follow-up at forty-seven years that 16.7% of those who had initially recovered experienced recurrence of PTSD during the COVID-19 pandemic.

Prevalence of PTSD recurrence in civilian adult populations

Fourteen studies focused on civilian adults. Findings relating to recurrence prevalence in entire trauma-exposed samples were mixed. Two studies reported rates of under 5% [ 45 , 58 ] in survivors of a terrorist attack and an earthquake respectively. Sungur and Kaya [ 64 ] reported a recurrence rate of 8.9% in survivors of the Sivas disaster, a religious fundamentalist protest which resulted in civilian deaths. Higher rates of recurrence were reported for survivors of an oil rig disaster (18.8%) [ 15 ], survivors of an oil spill (32%) [ 56 ], acutely injured trauma survivors (35%) [ 55 ] and women recently diagnosed with ovarian cancer (57%) [ 43 ].

For populations of civilians with PTSD only, recurrence rates were 4.9% [ 39 ] (type of trauma not reported), 15.4% [ 66 ] (trauma type varied), and 28% [ 46 ] (participants severely injured in accidents). Four studies reported data on the prevalence of recurrence in populations who had previously recovered from PTSD. Reported rates were 14% [ 52 ] (trauma type varied), 29.5% [ 37 ] (trauma type varied), 34% [ 35 ] (trauma type not reported) and 40% [ 65 ] (trauma type varied).

Prevalence of PTSD recurrence in children

Four studies focused on recurrence in adolescents / children, with mixed findings. Fan et al. [ 42 ] found that 3.3% of 1,573 earthquake survivors experienced ‘relapsing/remitting’ PTSD. Liang et al. [ 49 , 50 ] found that 17.7% of 301 earthquake survivors experienced the ‘relapsing’ trajectory of PTSD. An et al. [ 33 ] found that 37% of 246 adolescents experienced ‘recurrent dysfunction’ after experiencing an earthquake.

Prevalence of PTSD recurrence in combined military and civilian populations

Finally, two studies included both military and civilian participants; both of these studies were trials comparing fluoxetine to placebo treatment in people with PTSD. Davidson et al. [ 40 ] found that half of the placebo group relapsed after recovery, compared to 22.2% of the fluoxetine group. Martenyi et al. [ 53 ] reported lower rates of ‘relapse’: 16.1% of the placebo group and 5.8% of the fluoxetine group. The latter study followed up participants after 36 months, while Davidson et al. [ 40 ] followed up participants for a year after treatment.

Predictors of PTSD recurrence

The third and final aim of the present review was to identify factors associated with PTSD recurrence. Firstly, we note that (as shown in Table  2 ), participants in a number of studies had received some type of intervention during the study period, which was typically not accounted for in analyses of predictors. Many other studies did not report whether participants received treatment or not. Having treatment, whether it be medication, therapy, or a combination, is likely to be an important factor influencing PTSD trajectory, given that there are evidence-based treatments for the condition [ 68 ], but this was typically not explored.

Table  5 shows the factors considered as predictors in each study, with significant associations presented in bold. The majority of included studies (22/35) explored at least one covariate; the remaining studies either did not explore covariates or combined recurrent trajectories with other trajectories in their analyses of predictors. Of those studies which did explore covariates of recurrence, we found little consensus.

Sociodemographic factors

Gender was considered as a potential covariate by six studies; one [ 33 ] found that recurrent PTSD was associated with female gender while five studies (including two based on the same data-set) [ 49 , 50 ] found no significant gender association [ 35 , 36 , 42 , 49 , 50 ]. None of the three studies testing age as a covariate found a significant association [ 35 , 36 , 57 ]. One study of school-aged children found that children in a higher grade (i.e. older in age) were more likely to experience PTSD recurrence [ 33 ], while three studies of two cohorts [ 42 , 49 , 50 ] found no significant association between recurrence and school grade. Three studies considered race as a covariate, finding no significant association between PTSD recurrence and race [ 36 , 44 , 55 ]. Other socio-demographic characteristics considered included number of children in the family [ 42 ], marital status and level of education [ 36 ], none of which were found to be associated with PTSD recurrence. For military participants, there were no significant differences in service branch, service component or pay grade between the recurrent and rapid recovery groups [ 36 ].

Psychiatric history

Seven studies considered psychiatric history and concurrent diagnoses as potential covariates of PTSD recurrence, again with mixed findings. Recurrence was not found to be associated with other anxiety syndromes [ 36 ], baseline levels of anxiety [ 54 ], depressive symptoms [ 55 ], baseline levels of depression [ 54 ] or psychiatric history [ 55 ]. Ansell et al. [ 35 ] found that diagnoses of a number of co-morbid mental health disorders such as major depressive disorder and personality disorders such as schizotypal personality disorder, avoidant personality disorder and borderline personality disorder were not associated with recurrence, but participants with a baseline diagnosis of obsessive-compulsive personality disorder were significantly less likely to experience PTSD recurrence. Conversely, Perconte et al. [ 57 ] found that those who experienced recurrence were significantly more likely to report obsessive-compulsive symptoms than those whose symptoms improved without recurrence. Sakuma et al. [ 58 ] found that pre-disaster treatment for mental illness was significantly associated with PTSD recurrence, but note that the results should be interpreted carefully due to the very small number of participants in the ‘fluctuating symptoms’ group who appeared to have experienced recurrent episodes. Perconte et al. [ 57 ] found that, versus the improved symptoms group, those with PTSD recurrence were more likely to report depression, anxiety, hostility, phobic anxiety, somaticism and psychoticism; however, previous psychiatric hospitalisations and pre-treatment ratings of global pathology on a psychiatric scale did not predict recurrence. Finally, Madsen et al. [ 51 ] found that suicidal ideation was significantly higher in the ‘relieved-worsening PTSD’ group than the ‘low-stable’ group and that suicidal ideation was in fact highest in the recurrent (termed ‘relieved-worsening’) group than any other. However, it should be noted that suicidality was not assessed at baseline in this study, therefore it is not clear whether suicidal ideation is a cause or a consequence of PTSD recurrence.

Physical health

Fewer studies considered physical health as a potential predictor of PTSD recurrence. One study found no association between recurrence and disabling injury/illness, somatic symptoms or bodily pain [ 36 ] and another found no association between recurrence and prior treatment for physical illness [ 57 ]. However, obesity was a significant predictor of PTSD recurrence [ 36 ]. In terms of health-related behaviours, Armenta et al. [ 36 ] found no association between PTSD recurrence and smoking status, alcohol problems or sleep duration. However, Perconte et al. [ 57 ] found that higher weekly alcohol intake both before and at termination of PTSD treatment predicted recurrence.

Cognitive ability

Only one study [ 63 ] explored cognitive ability as a potential covariate, finding that the participants who were in the recurrent (termed ‘relieved-worsening PTSD’) group had significantly lower cognitive ability scores than those in the ‘low-stable’ group.

Trauma history and pre-trauma experiences

The review also found mixed evidence for trauma history as a predictor of PTSD recurrence. Liang et al. [ 49 , 50 ] found no association between pre-disaster traumatic experience and PTSD recurrence. Armenta et al. [ 36 ] found no association between recurrence and childhood sexual abuse, childhood verbal abuse, childhood neglect, sexual assault, physical assault, or ‘other life events’, but did find that participants reporting a history of childhood physical abuse were significantly more likely to experience PTSD recurrence. Holliday et al. [ 47 ] found that veterans who had experienced military sexual trauma (MST) had greater initial reductions in PTSD symptoms than those who had not experienced MST, but also experienced a ‘modestly greater’ recurrence of symptoms than those without MST, although this difference did not appear to reach statistical significance. Zanarini et al. [ 65 ] found that the presence of childhood sexual abuse history did not significantly predict time-to-recurrence, but severity of childhood sexual abuse, adult rape history, combination of childhood sexual abuse history and adult rape history, and experiencing sexual assault during study follow-up were associated with less time-to-recurrence. Osofsky et al. [ 56 ] found that abuse, emotional abuse, domestic violence, and greater number of traumas experienced were associated with recurrence of PTSD, and Osenbach et al. [ 55 ] found that recurrent life stressors significantly increased the odds of membership in chronic, relapsing or recovery groups rather than the resilient group. For military participants, one study found combat deployment was significantly associated with recurrent PTSD [ 36 ] while others found combat exposure was not associated with recurrence [ 54 , 57 ]. Finally, Fan et al. [ 42 ] found that compared to the recovery group, relapsing participants experienced significantly fewer negative life events 6-months post-disaster, but significantly more such events at the 24-month follow-up.

Few other pre-trauma experiences were considered. An et al. [ 33 ] found that those with recurrent PTSD were significantly more likely to have experienced academic burnout than those in the recovery trajectory, although there was no difference between the recurrent and delayed trajectories.

Experiences during and immediately after the traumatic experience

The review also found mixed evidence for an association between peri-traumatic experiences and PTSD recurrence. The most consistent finding related to how stressful the traumatic experience was perceived to be at the time. For example, risk of recurrence was significantly higher in those with combat stress reactions [ 59 ] and in those with higher stress relating to the disaster they had experienced [ 56 ], as well as with greater trauma severity [ 49 , 50 ]. However, recurrence was not found to be associated with subjective fear during the event [ 33 ]; directly witnessing a disaster [ 42 ]; property loss during the event [ 33 , 42 ]; property damage [ 42 ]; displacement due to property damage [ 58 ]; near-death experience [ 58 ]; or having a family member injured, killed or missing [ 42 , 58 ].

There was some evidence that initial post-traumatic stress symptoms immediately after the traumatic event could predict PTSD trajectory. Liang et al. [ 49 , 50 ], in a study of PTSD in children from two schools affected by an earthquake, found that children from one of the two schools (‘School 2’) were significantly more likely to experience PTSD recurrence than children from the other school (‘School 1’). Further investigations revealed that after adjusting for immediate post-traumatic stress symptoms the school no longer predicted relapse; those from School 2 had significantly greater post-traumatic stress symptoms immediately after the disaster, which the authors suggest might be due to School 1 providing sufficient psychological services as well as having the same students and teachers before and after the earthquake (therefore perhaps greater social support available), whereas School 2 had insufficient psychological services and consisted of teachers and students from several different schools which could not be reconstructed after the earthquake.

One study [ 58 ] considered occupational-related covariates of PTSD recurrence for disaster recovery workers. They found that having mainly disaster-related occupational duties and lack of rest due to occupational duties were not associated with recurrence, but perceived poor workplace communication did predict recurrence.

Post-trauma experiences and symptoms

An et al. [ 33 ] found that, compared to the delayed PTSD trajectory, those who experienced recurrence were less likely to have experienced post-traumatic growth after the traumatic event; however, there were no differences in post-traumatic growth between the recurrent and recovery groups. Fan et al. [ 42 ] found that neither positive coping nor negative coping six months post-disaster were associated with PTSD recurrence. In a military study, Karstoft et al. [ 48 ] found that poor adjustment to civilian life (i.e. difficulties with community reintegration after deployment) was significantly higher for the recurrent (‘relieved-worsening PTSD’) group than all other groups. However, it is not clear whether poor adjustment was a cause or an effect of PTSD symptoms worsening after initial improvement.

Two studies explored specific cluster symptoms. Murphy and Smith [ 54 ] found PTSD recurrence was not predicted by the magnitude of re-experiencing, avoidance, or hyperarousal symptoms. Boe et al. [ 15 ] found that the number of intrusion and avoidance symptoms five-and-a-half months post-trauma did not predict recurrence, but the number of intrusion and avoidance symptoms both fourteen months and five years after the disaster did predict recurrence.

Social support

Only three studies directly considered social support as a potential covariate. Armenta et al. [ 36 ] found no association between social support and PTSD recurrence, and Perconte et al. [ 57 ] found that family support did not predict recurrence. Fan et al. [ 42 ] found that level of social support six months after experiencing an earthquake was not associated with PTSD recurrence, but those in the ‘relapsing’ group reported significantly less social support 24 months after the earthquake than those in the ‘recovery’ group.

PTSD treatment

Most of the studies investigating treatment for PTSD found that not receiving interventions, or discontinuing treatment, were associated with PTSD recurrence. For example, Osenbach et al. [ 55 ] found that those who received ‘usual care’ only were significantly more likely to experience recurrence than those who received interventions designed to reduce post-traumatic symptoms. Davidson et al. [ 40 ] found that those who received placebo treatment were significantly more likely to experience recurrence than those who received fluoxetine. Martenyi et al. [ 53 ] found that those who discontinued fluoxetine treatment were significantly more likely to experience recurrence, especially for those with combat-related PTSD. However, Perconte et al. [ 57 ] found that number of weeks enrolled in treatment and number of treatment sessions attended did not significantly affect risk of recurrence. In this study, though, being hospitalised at least once since the termination of treatment was used as a proxy measure of ‘recurrence’ and so the findings are arguably not truly representative of actual recurrent episodes of PTSD. Overall, our findings indicated some evidence that treatment helped to avoid recurrent episodes.

In this study, we systematically reviewed 35 studies to identify definitions and prevalence of recurrent PTSD and factors associated with recurrence. It is important to define and operationalise recurrence as the concept needs to be understood in order to make prevention efforts. The health-related, social and economic costs of PTSD can be substantial. PTSD negatively affects individuals’ emotional wellbeing and physical health [ 7 ], impedes social relationships [ 69 ], limits productivity at work and increases sickness absence [ 70 ]. The direct costs (e.g., medical care costs) and indirect costs (e.g., costs of unemployment or reduced productivity) of PTSD can create substantial economic burden [ 7 , 71 ]. Determining the predictors of recurrence of PTSD (which can only be properly understood if ‘recurrence’ itself has a clear definition) is important for prevention efforts: identifying those most at risk for recurrent episodes would allow for the subsequent investigation of ways of mitigating or preventing the risk. However, we found little consensus as to how recurrence is defined, mixed evidence on the prevalence of recurrence and inconsistent findings relating to predictors of recurrence. This lack of clarity about what relapse or recurrence is, and is not, is a major barrier to understanding this important topic.

In a previous review exploring PTSD recurrence in veterans, Berge et al. [ 22 ] acknowledge that there is no generally accepted or used definition of recovery relating to psychological trauma. The definition of recurrence used in their review was the return of symptoms following a period of complete recovery, representing the start of a new and separate episode . However, it is not clear what length of time is covered by ‘a period of complete recovery’ nor what ‘complete recovery’ means. How many days, weeks, or months does an individual need to be free of symptoms of PTSD in order to be considered truly recovered? Is ‘symptom-free’ the only definition of recovery, or is ‘not meeting the criteria for PTSD’ enough? Our own review revealed that there is little consensus as to what recurrence means and the parameters for its definition. Even the terminology used varied across studies, with ‘relapse’, ‘recurrence’, ‘reactivation’ and numerous other terms often used to describe what essentially appeared to be the same concept. There was no consensus as to how long an individual needed to be free of symptoms in order to be considered recovered, nor for how long symptoms needed to recur in order to be considered a recurrent episode. Most studies simply defined recurrence as a change in symptoms between assessments, meaning that whether or not an individual was defined as having a recurrent episode or not very much depended on the scores they reported at arbitrary time-points. Even minor symptom fluctuations could cause someone to change from being identified as a ‘case’ to ‘recovered’ and vice versa. Because PTSD tended to be examined using prospective studies where symptoms were assessed at predetermined assessment points, it is possible that individuals may have onsets of PTSD after one assessment and then remit before the next. With no retrospective assessment between time-points, it is difficult to assess the true prevalence of recurrence. Andrews et al. [ 16 ] make a similar point in relation to delayed onset PTSD, suggesting the absence of information about symptoms outside of the predetermined time-points of studies means that estimates of delayed onset PTSD may be unreliable.

The second aim of the review was to examine the prevalence of PTSD recurrence in existing literature. Given the numerous different ways of assessing PTSD, defining initial recovery and defining recurrence, as well as the differing time-points at which PTSD was assessed across studies, we suggest that the current data on recurrence prevalence is not especially meaningful. We found very different prevalence rates reported within the literature, with data suggesting that anywhere between 0.2% and 57% of trauma-exposed populations might experience recurrent episodes of PTSD. Some of the higher percentages we found seem greater than we would expect, given that only a minority of trauma-exposed people are likely to develop PTSD in the first place – let alone suffer from it, recover from it, and experience a recurrent episode. We would expect that studies carried out over a longer period of time would find higher recurrence rates, simply because in these studies there is more time for recurrent episodes to occur. However, the highest prevalence rate (57%) was found in a study which took place over only 27 weeks [ 43 ]; the authors labelled these participants as ‘intermittent cases’ and it appears likely that symptom fluctuation, rather than true recovery and recurrence, occurred in this study – and potentially many others. Additionally, studies did not typically control for exposure to subsequent trauma, meaning that ‘recurrences’ of PTSD identified may actually be new episodes, rather than a relapse. Further research studies, especially research involving assessments over a number of years, are needed to establish the true prevalence of recurrent PTSD which also needs to be clearly defined with an agreed time period between remission and relapse.

It has been proposed that recurrence rates might increase with old age. Murray [ 72 ] suggests that PTSD can be ‘reactivated’ in older age because physical illnesses become more common, which can reactivate traumatic memories; increased dependence on others due to ageing can reactivate feelings of helplessness; and loss of structure and identity caused by retirement can similarly reactivate traumatic symptoms. Other factors relating to ageing such as decline of cognitive function, difficulty controlling ruminations, reminiscing, and late-life stressors such as serious illness, surgical procedures and death of spouses, siblings or close friends can either directly remind the person of their previous traumatic experience(s) or can induce similar feelings of vulnerability [ 73 ]. Three studies of adults in this review did not find age predicted recurrence [ 35 , 36 , 57 ]; however, the populations trended young overall, with each of the three studies reporting the mean age of participants was under 40. We suggest, then, that more studies of older adults with lifetime PTSD are needed to establish whether this group are at increased risk of recurrence.

The third aim of this review was to understand factors associated with PTSD recurrence. Although a number of potential covariates were considered, most were not investigated by more than a few studies, and findings were varied and inconsistent. Of the covariates investigated by multiple studies, none were found to have significant associations with recurrence across all studies. It was therefore not possible to quantify the extent to which potential risk factors contribute to the risk of recurrence. One reason for the inconsistent findings might be the relatively small numbers of participants with recurrent PTSD in many of the studies. We note also that most studies did not consider either subsequent trauma or treatment impact in their analysis of predictors of recurrence.

We did not find strong evidence of an association between PTSD recurrence and comorbid psychiatric conditions. Recurrence of other mental health disorders, such as anxiety, is reportedly associated with comorbid psychiatric conditions including major depression, alcohol and substance use disorders [ 74 ]. Additionally, comorbid disorders have been found to be associated with an ‘unfavourable long-term course’ of PTSD [ 18 ]. However, in a review of predictors of developing PTSD, Brewin et al. [ 75 ] found that while psychiatric history was associated with development of PTSD, it was not a strong risk factor – factors operating during or after the traumatic exposure had greater effects than the pre-trauma factors. Many studies in this review found no evidence of a relationship between PTSD recurrence and other mental health conditions; in those that did find a relationship, it was not always clear whether the other conditions pre-dated the recurrent PTSD episode or not. Overall, the most consistent evidence we found indicated that recurrence of PTSD was associated with greater stress and traumatic response at the time of the traumatic experience.

We did not find evidence to suggest that trauma type may affect recurrence. Many studies examined PTSD trajectories after a single traumatic event. Those that did include participants who had experienced various different types of trauma did not consider trauma type as a potential predictor of recurrence. Given the wide variations in methodology, it was not appropriate for us to compare recurrence rates for different trauma types within the review. Future research should include participants who have experienced different types of trauma and should consider trauma type as a potential predictor of PTSD trajectory.

Only one study assessed PTSD during the COVID-19 pandemic, with Solomon et al. [ 62 ] reporting that 16.7% of initially-recovered participants experienced recurrence during the pandemic. However, it is not clear how many of this cohort may also have experienced recurrence before the pandemic, and without being able to make that comparison, we cannot ascertain the extent to which recurrence was exacerbated by the pandemic. Additionally, the percentage (16.7%) is similar to recurrence rates in several other, non-COVID studies. Ideally, future studies will present data on PTSD recurrence rates for one cohort at regular intervals, including data collected during or after the COVID-19 pandemic, to ascertain whether the pandemic did affect recurrence rates.

In their review, Steinert et al. [ 18 ] identified older age, higher education, greater trauma severity, higher baseline symptoms, more physical/functional impairments, and poorer social support as predictors of ‘unfavourable’ long-term course of PTSD. These were identified as predictors due to being reported in at least two studies within their review. The current review did not find consistent evidence that age, education, trauma severity, baseline symptoms, impairments or social support predicted recurrence – although age was only considered in studies of young people. We found some evidence from treatment studies that fluoxetine reduced the risk of recurrence, as did participation in an intervention involving a combination of motivational interviewing, behavioural activation and pharmacotherapy. It is therefore difficult to make recommendations relevant to occupational health, as we had hoped to do. Managers of trauma-exposed employees who have developed PTSD may have questions around whether recovered individuals can go back to frontline work, or whether they risk experiencing a recurrence of PTSD. Our findings tentatively suggest that recurrence might be relatively rare (rates of recurrence ranged from 0.2 − 57% in full trauma-exposed samples, mean 13.1%; 4.9 − 49.6% in PTSD-only subgroups, mean 24.5%; and 5.8 − 50% for recovered subgroups, mean 25.4%) but clearer definitions and assessments of recurrence are needed to substantiate that claim. As we found no consistent evidence of predictors of recurrence, it was therefore not possible to identify which sub-groups of people might be more likely to have their PTSD recur. We did find evidence from two studies that recurrence was more prevalent in groups of PTSD patients treated with placebos compared to PTSD patients treated with fluoxetine, suggesting that medication appears at least somewhat effective in reducing the risk of recurrence. However, we found no studies looking at the impact of first-line treatments on relapse (i.e. trauma-focused cognitive behavioural therapy [ 76 ] or eye movement desensitisation and reprocessing [ 77 , 78 ]) which is a major gap in the literature. Whilst more, high-quality studies are carried out, employers should ensure that workers get evidence-based treatments and have an occupational mental health assessment on completion of potentially traumatic work to provide an expert judgement, given that we cannot identify any clear risk factors from the literature.

The key limitation of the literature on PTSD recurrence is that it is not always easy to differentiate between recurrence and symptom fluctuation, and it is also difficult to know what ‘recovery’ truly means. It is not clear how many of the so-called ‘recovered’ participants within the reviewed studies may have been close to clinical thresholds for PTSD at the assessment points. Rather than moving from distinct ‘recovered’ to ‘recurrent episodes’, it may be that individuals only experienced small fluctuations in PTSD symptoms, moving them above and below the symptom thresholds. Indeed, the authors of several of the included studies remarked on the difficulties in identifying PTSD trajectories. In Boe et al.’s [ 15 ] study, clinical interviews were conducted by two clinical psychologists who were trained and supervised by an experienced clinician and trauma researcher and even these experienced individuals had difficulties identifying recurrence of PTSD, with one case being recategorised from ‘full-blown PTSD reactivation’ to ‘sub-syndromal reactivation’ after discussion between the researchers. Markowitz et al. [ 52 ] pointed out that, as they defined relapse as ‘loss of response (to treatment) status’, relapse might reflect barely crossing that threshold: indeed, more in-depth analysis of their six ‘relapsers’ showed that all but one still showed some, albeit more modest, treatment benefit relative to their baseline PTSD severity.

Sakuma et al. [ 58 ] discussed their finding of a ‘fluctuating’ trajectory (and lack of a delayed-onset trajectory), differing from the typical four trajectories widely accepted within the PTSD literature. They suggested the difference may be due to variations in the duration of study periods and characteristics of the study samples. The majority of studies which produce the typical four trajectories are conducted over short periods between a few months and two years [ 9 ], compared to the longer (54-month) period of Sakuma et al.’s [ 58 ] study: the trajectory commonly identified as ‘delayed onset’ could really be a fluctuating trajectory if examined over a longer period. Or, it could reflect a gradual accumulation of symptoms resulting in a delayed presentation of PTSD, rather than delayed onset.

The time-points of assessments could also affect reported prevalence rates. For example, Sungur & Kaya [ 64 ] pointed out that some of their ‘recurrent’ cases would have been considered ‘recovered’ if the study period had been shorter or if participants had not been reassessed at the particular time-points chosen. They also noted that symptoms across the entire participant population seemed to be higher at particular times during the study (namely, at the anniversary of the event and at the time of a disappointing result of a court hearing for compensation), suggesting that the nature and course of PTSD might be influenced by particular events which might trigger unwanted memories of the traumatic event. In the current review, most studies assessed participants for at least a year, but not all: five [ 38 , 39 , 43 , 52 , 53 ] followed participants for less than a year. Additionally, two studies [ 44 , 47 ] reported assessing participants pre-treatment and four months post-treatment but it was not clear how long treatment lasted.

We suggest that PTSD recurrence may not have been adequately assessed in many of the included studies. For example, Chopra et al. [ 39 ] described how, in order to minimise respondent burden, assessors were expected to stop inquiring about PTSD symptoms if participants were unlikely to meet the criteria and if they answered no to particular questions on the assessment tool. This could mean that some individuals who did have recurrent episodes of PTSD were not identified as they did not complete the full measures. Additionally, we found that a number of studies had very vague definitions of recurrence, such as ‘increasing symptoms’, where it was unclear what exactly this meant. Others used hospitalisation as a proxy measure for recurrence, or simply asked participants whether they perceived their symptoms had been exacerbated and in one case used the investigator’s own judgements as a way of determining recurrence. It is therefore likely that some recurrent cases may have been missed while others who never truly ‘recovered’ at all may have been reported to have experienced recurrence. Overall, the vague and inconsistent ways of assessing recurrence mean it is currently impossible to ascertain true recurrence rates within existing literature.

It is also possible that recurrent trajectories of PTSD appear in studies which do not identify them as such. For example, in Andrews et al.’s [ 16 ] review, the authors note that some cases of ‘delayed-onset PTSD’ in veterans of relatively old age with long intervals to first onset may in fact have had episodes of PTSD soon after their traumatic experiences which were undisclosed or forgotten. In other words, some cases of supposedly ‘delayed-onset’ PTSD might actually be recurrent cases. Andrews et al. [ 16 ] also point out that many of the studies included in their review of delayed-onset PTSD did not assess whether respondents could have had onsets of PTSD and then remitted before the next assessment point – which could lead to both over- and under-estimates of delayed-onset rates of PTSD. Indeed, the studies included in our own review tended to focus only on the scores at the various time-points and did not explore participants’ perceptions of symptom fluctuations outside of the time-points set by the study.

Limitations

There are a number of limitations of the literature reviewed. Many did not collect data on whether participants had undergone any intervention or not, and those that did tended not to include this as a potential confounding variable. The majority of studies did not assess whether participants experienced additional potentially traumatic experiences between PTSD assessments. Many did not define the parameters of ‘recovery’ and ‘recurrence’ and it is not clear whether recurrent episodes identified were truly recurrent episodes or merely symptom fluctuations. Many did not collect data on whether or not participants received any treatment for PTSD between data collection time-points, and many of those which did ask participants whether they had received any treatment did not distinguish between types of treatment. It is therefore unclear if, and how many, participants in many studies received any evidence-based PTSD treatment or not. Additionally, the majority of studies did not collect data on the time period of any treatment received. Some studies had extremely long gaps (e.g., decades) between assessments which could mean that recurrences were missed.

There are also limitations of the review process itself. Firstly, the screening, data extraction and quality appraisal were carried out by one author. Although decisions about exclusion or inclusion were discussed with the second author, it would have been preferable to have multiple screeners. We limited the review to English-language studies only, meaning that important studies published in other languages would have been missed. We included only studies which identified ‘recurrent episodes’ (or equivalent terminology e.g. relapse, reactivation); studies which identified no recurrent trajectory were not reviewed. It may be that these studies did not include a sufficient number of assessments to pick up on recurrent episodes, but it may also be that no participants in these studies experienced recurrence and therefore the true prevalence of recurrence may be lower than this review suggests.

Conclusions and implications

The main conclusion that can be drawn from the current review is that, moving forward, better clarity and consensus regarding the definition and identification of recurrent PTSD are urgently needed. Berge et al. [ 22 ] suggest that consistent definitions of relapse-related terms, supported by empirical research, are required in order to make studies of PTSD trajectories more robust. The findings of this review support this suggestion. Experts in the field should agree on an appropriate definition of recurrence (i.e. symptom-based or threshold-based) and should agree how long an individual needs to be ‘better’ for in order to be considered recovered as well as how long an individual needs to experience symptoms for in order to be considered as having a recurrent episode. Recurrence is arguably better-defined for recurrent depressive disorder, with the ICD-11 stating that recurrence is characterised by a history of depressive episodes separated by at least several months without significant mood disturbance [ 79 ]. However, further clarity is still needed. How many months is ‘several’? What are ‘significant’ symptoms? Still, we suggest this might be a useful starting point for a working definition of recurrent PTSD: a history of episodes of PTSD separated by at least several (i.e., three) months without significant (i.e., meeting diagnostic criteria) PTSD symptoms . However, further research is necessary to clarify whether these parameters (i.e. three months as a time period, symptom thresholds as a diagnostic tool) are the most appropriate to use. Using consistent terminology within the literature would make it easier to researchers in the future to understand true prevalence rates of PTSD recurrence and to compare them across studies. Further research allowing for the identification of recurrent PTSD episodes is needed. We believe the gold standard for assessing PTSD and properly identifying its trajectories, including recurrent trajectories, would be using the Clinician Administered PTSD Scale (CAPS) [ 80 ], or other validated questionnaires, at multiple specific time points over a long period of time. Figure  2 summarises the findings of the review and the proposed next steps based on our findings.

Summary of review and suggested next steps

It is important to understand recurrence in order to take steps towards reducing the risk of PTSD recurring. However, due to the inconsistent findings relating to predictors of recurrence, it is difficult to draw conclusions about the best ways of preventing or minimising recurrence. We suggest that ensuring that people who develop PTSD are provided with timely, evidence-based treatments is a logical first step [ 68 ]. Second, awareness of ‘early warning sign’ symptoms and ‘triggers’ might be useful, as well as awareness of effective coping strategies and how to access support. That is, if people with PTSD are able to recognise when they are struggling more and acknowledge that they need to be proactive in ensuring symptoms do not develop into full-blown PTSD again, they may be able to draw on their coping skills or reach out for formal or informal support when a recurrent episode seems imminent and may be able to stave off the recurrent episode. We also suggest that reframing the re-emergence of symptoms in a more positive way might be useful: instead of feeling defeated that symptoms have recurred, people could remind themselves that they have recovered once and therefore know that they are capable of doing so again. Within organisational settings, it is also important to foster an environment in which people who have any mental health condition, including PTSD, feel confident that asking for help will not lead to stigmatisation or increase the likelihood of inappropriate job loss. It may also be helpful to incorporate relapse prevention, understanding ‘warning signs’ of recurrent episodes and positive reframing into PTSD treatment programmes.

Data availability

All data generated or analysed during this study are included in this published article.

Abbreviations

Clinician-Administered PTSD Scale

Military sexual trauma

National Institutes for Health

Preferred Reporting Items for Systematic Reviews and Meta-Analyses

Post-Traumatic Stress Disorder

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This study was funded by the National Institute for Health and Care Research Health Protection Research Unit (NIHR HPRU) in Emergency Preparedness and Response, a partnership between the UK Health Security Agency, King’s College London and the University of East Anglia. The views expressed are those of the author(s) and not necessarily those of the NIHR, UKHSA or the Department of Health and Social Care. For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising. The funders had no role in carrying out the review or preparing the manuscript for publication.

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Brooks, S.K., Greenberg, N. Recurrence of post-traumatic stress disorder: systematic review of definitions, prevalence and predictors. BMC Psychiatry 24 , 37 (2024). https://doi.org/10.1186/s12888-023-05460-x

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REVIEW article

Important soil microbiota's effects on plants and soils: a comprehensive 30-year systematic literature review.

• School of Karst Science, State Engineering Technology Institute for Karst Desertification Control, Guizhou Normal University, Guiyang, China

Clarifying the relationship between soil microorganisms and the plant-soil system is crucial for encouraging the sustainable development of ecosystems, as soil microorganisms serve a variety of functional roles in the plant-soil system. In this work, the influence mechanisms of significant soil microbial groups on the plant-soil system and their applications in environmental remediation over the previous 30 years were reviewed using a systematic literature review (SLR) methodology. The findings demonstrated that: (1) There has been a general upward trend in the number of publications on significant microorganisms, including bacteria, fungi, and archaea. (2) Bacteria and fungi influence soil development and plant growth through organic matter decomposition, nitrogen, phosphorus, and potassium element dissolution, symbiotic relationships, plant growth hormone production, pathogen inhibition, and plant resistance induction. Archaea aid in the growth of plants by breaking down low-molecular-weight organic matter, participating in element cycles, producing plant growth hormones, and suppressing infections. (3) Microorganism principles are utilized in soil remediation, biofertilizer production, denitrification, and phosphorus removal, effectively reducing environmental pollution, preventing soil pathogen invasion, protecting vegetation health, and promoting plant growth. The three important microbial groups collectively regulate the plant-soil ecosystem and help maintain its relative stability. This work systematically summarizes the principles of important microbial groups influence plant-soil systems, providing a theoretical reference for how to control soil microbes in order to restore damaged ecosystems and enhance ecosystem resilience in the future.

1 Introduction

The principle of microbial remediation of the environment is a topic that people have always been working on researching ( White et al., 1998 ; Cruz et al., 2017 ; Sharma et al., 2022 ; Verma et al., 2023 ; Torres-Farradá et al., 2024 ). Utilizing microbial remediation techniques can not only clean and efficiently reduce environmental pollution but also restore and rebuild ecosystems, maintaining ecosystem stability ( Singh et al., 2019 ; Lu et al., 2023 ; Saeed et al., 2023 ). As high-throughput sequencing technology and molecular information technology increasingly develop ( Qiang-long et al., 2014 ; Gray et al., 2015 ), a lot of new microbial strain species have been found and chosen. By studying their structure and function, we can predict how these microbial strain work to restore ecosystems and clean environment ( Liu et al., 2020a ; Fernandez Nuñez et al., 2021 ; Xiao et al., 2021 ). After analyzing and summarizing current research findings, it can be found that bacteria, fungi, and archaea are important components of the soil microbial community ( Fierer, 2017 ). However, in studies aimed at environmental remediation, the knowledge on how important microbial groups exert their effects is relatively fragmented, and the research content is not detailed enough. It is worth nothing that elucidating the mechanisms by which important soil microbes exert their effects is of significant reference value for restoring damaged ecosystems. Moreover, as the main targets of microbial activity in the soil, the changes in plants and soil will reflect the level and quality of microbial environmental remediation capabilities ( Ma and You, 2016 ; Zhao et al., 2021 ). Thus, exploring the structure, physiological activities, and impact mechanisms of important microbial groups at the phylum and genus levels can clarify the complex relationships among microbes, plants, and soil. Through using the right and appropriate mechanisms, people can improve the resilience of soil ecosystems against disturbances, increase their functionality, reduce the harm that pollutants cause, and better maintain the stability of ecosystems.

According to Nemergut et al. (2013) , soil microbial communities are among the most abundant and diverse biological groups in nature. A rich and varied microbial population, comprising bacteria (including actinomycetes), fungi, viruses, archaea, algae, and protozoa, can be found in one gram of soil ( Pepper and Gentry, 2015 ; Islam et al., 2020 ; Sokol et al., 2022 ). Among them, bacteria, fungi, and archaea are important components of soil microorganisms ( Bayranvand et al., 2021 ). With 70–90% of the total biomass in the soil, bacteria are the most prevalent microorganisms. Fungi are second only to bacteria in abundance. Both are engaged in nearly all ecological processes that impact soil and plants. Archaea—which inhabit extreme environments—make up a substantial fraction of soil microorganisms. Due to their distinct structural features, they are referred to as the third form of life ( Allers and Mevarech, 2005 ; Ibáñez de Aldecoa et al., 2017 ). It is helpful to promote further research on microbial diversity based on studying the way archaea affect plants and soil. As opposed to bacteria, fungus, and archaea, algae and protozoa are much less common and have less of an effect on soil and plants ( Devi and Soni, 2019 ). In the plant-soil underground ecosystem, soil microorganisms mainly establish connections with plants in the following three ways ( Van Der Heijden et al., 2008 ): (1) plant residues, including roots, leaves, and other secretions, are the primary source of soil carbon to supply microbes in the soil ( Zhang et al., 2022a ); (2) soil microorganisms release nutrients through metabolic activities, supporting soil development, promoting plant growth, and maintaining the stability of soil carbon cycling; (3) through mycorrhizal symbiosis ( Hayat et al., 2010 ), releasing hormones, and stress signals ( López-Bucio et al., 2007 ), microorganisms can directly affect plant growth and development. On the other hand, microorganisms can establish soil structure and form soil aggregates through gas exchange and the production of organic acids ( Ahmed et al., 2021 ). Microorganisms can also increase soil fertility by breaking down organic matter and minerals, releasing nutrients, and releasing inorganic compounds ( Wang et al., 2021a ), as seen in Figure 1 . Thus, microorganisms are involved in influencing the ecological processes that affect plants and soil, maintaining the stability of the plant-soil ecosystem ( Huet et al., 2023 ).

Figure 1 . Schematic diagram of the relationship between soil microorganisms, plants, and soil.

Diverse soil microbial groups affect plants and soil through different mechanisms. Common soil bacteria and fungi play important roles in decomposing organic matter, suppressing the growth of pathogens, promoting nutrient cycling, and improving soil texture and structure. For example, a type of bacteria called Bacillus subtilis is in the Firmicutes phylum. It can make bacillomycin, organic acids, and antibacterial proteins that stop pathogens from spreading and growing ( Zhang et al., 2022b ). Another example is the Rhizobium genus, which belongs to the Proteobacteria phylum and changes nitrogen gas into ammonia or nitrite that plants can use. It does this by fixing nitrogen, which helps plants grow and encourages nitrogen cycling ( Sharma et al., 1993 ; del Carmen Orozco-Mosqueda et al., 2013 ; Saidi et al., 2021 ). Moreover, Pseudomonas , a genus within the Proteobacteria phylum, secretes organic acids, which contribute to improving soil structure and maintaining soil moisture ( Halverson et al., 2000 ; Archana et al., 2013 ). For soil fungi, Trichoderma is a genus of fungi in the Deuteromycotina subphylum that can stop pathogenic fungi and bacteria from growing on plants ( Marques et al., 2018 ). According to Chen et al. (2023) , arbuscular mycorrhizal fungi (AMF) in the Glomeromycota phylum live in a mutualistic symbiosis with plant roots. This helps plants get more nutrients and grow and develop. Meanwhile, AMF's mycelial network can also encourage soil particle aggregation, which helps to produce soil structure and enhances aeration and water retention in the soil ( Beare et al., 1997 ; Zhang et al., 2022c ). On the other hand, in order to preserve the health of the soil and the ecological balance, archaea are crucial to the decomposition of organic matter, the cycling of nitrogen, and the consumption of methane. For instance, methanogenic archaea (MA) and ammonia-oxidizing archaea (AOA) mediate the cycling of carbon, nitrogen, and other elements in the soil to maintain plant growth and health, and they belong to the Euryarchaeota and Thaumarchaeota phylums, respectively ( Jung et al., 2020 ; Ni et al., 2023 ). Compared to bacteria, fungi, and archaea, algae and protozoa have comparatively less of an effect on plants and soil because they have certain needs for their living conditions. Algae use photosynthesis to build up organic materials in the soil while they reside in damp surface soil. Research has demonstrated that cyanobacteria can fix nitrogen, which qualifies them as biofertilizers ( Ammar et al., 2022 ). Because they consume organic leftovers and decompose in soil that is rich in organic matter, protozoa have the ability to alter the makeup and abundance of other soil microbial communities ( Guo et al., 2022 ). These microbes have a unique living habitat, which makes their abundance variable and highly susceptible to outside influences. Therefore, in this study, algae and protozoa are not regarded as significant microbial groups. Understanding how the above important soil microbial groups affect plants and soil can help with managing them, which will promote plant growth, improve soil quality, and keep the ecosystem stable in the long run.

Numerous studies have been done on certain microorganisms that are found in the core root biome right now, like plant growth-promoting rhizobacteria (PGPR) and plant growth-promoting fungi (PGPF; Raho et al., 2022 ; Jernigan et al., 2023 ; Ramakrishnan et al., 2023 ). In addition, there is also a lot of research on the contributions of MA and AOA to the processes of carbon and nitrogen cycling ( Ke et al., 2014 ; Day et al., 2022 ). Despite some encouraging findings, it is still unclear how significant microbial communities affect plants and soil and how these processes are systematically understood. To fully understand the intricate interactions between soil, microbial groups, and plants, as well as the potential role that soil microbes may play in enhancing ecosystem stability and function, more research is required. Furthermore, there is still a lack of knowledge about the application of restoring ecosystems to important microbial groups' action mechanisms. Therefore, from the perspective of soil microorganisms, we use the systematic literature review (SLR) method to address the above issues. At the phylum and genus levels of important microbial groups, their mechanisms that affect plants and soil will be summarized, and the latest research progress will be reviewed. We will conduct a systematic review of global research progress on the impacts of important microbial groups on plants and soil from 1992 to 2022 by searching relevant literature in the Scopus database and performing quantitative analysis. By comprehensively analyzing advancements in this field, we aim to provide theoretical references for future research on utilizing microbial principles to restore damaged ecosystems.

2 Materials and methods

SLR is a method based on existing literature to study specific fields. It has the advantages of high relevance to the topic and clear predictions for future vision ( Tikito and Souissi, 2019 ). At the same time, it is an interdisciplinary approach that can be used to summarize the research status of specific topics ( Burgers et al., 2019 ). When searching for literature, entering the correct keywords in the search bar and determining the important steps of the search can minimize the loss of literature information. In this study, we adopted the method of systematic review to organize published literature and scientifically evaluate the impact of soil microorganisms on plants and soil, as shown in the flow framework in Figure 2 .

Figure 2 . SLR flow framework. Begin with Step 1 and end with Step 5.

2.1 Programs and plans

This study is based on articles from the Scopus database. Scopus is currently the largest abstract and citation database in the world, with the widest coverage, and it maximizes the search for published literature. The literature was screened based on the flow framework, and the research scope was determined based on the selected thematic literature to address research questions and summarize existing progress and research findings.

2.2 Identification and retrieval

The second step in the flow framework is the retrieval stage, where relevant articles on the impact of soil microorganisms on plants and soil worldwide are searched in the Scopus database. Keywords such as “soil microorganisms,” “plants,” and “soil” are entered in the search box, and Boolean logic operations are performed. After the initial search, a total of 28,972 records were obtained. Subsequently, review studies unrelated to research progress were excluded, and duplicate articles were removed, resulting in 17,217 articles. Next, abstracts were manually screened using keywords such as “effects, impact, actions, and mechanisms” to narrow down the selection, resulting in 1,959 articles as the preliminary screening results, as shown in Table 1 .

Table 1 . Searching the database for terms about the topic literature.

2.3 Screening and evaluation

One thousand nine hundred and fifty-nine articles were evaluated, and we manually selected literature relevant to our research topic in order to further narrow down the scope of the study. Inclusion and exclusion criteria were used as the basis for assessing the validity of the articles. The inclusion criteria are as follows: (1) The topic words “bacterium or bacteria,” “fungus or fungi,” “archaea,” or similar terms exist or at least appear in the titles, abstracts, or keywords of the literature. (2) The main subject of the research must be soil bacteria, fungi, or archaea. (3) The objects of the research must be plants and/or soil. (4) The study must be related to the mechanisms of the impact of important microbial groups such as bacteria, fungi, and archaea on plants and soil. The exclusion criteria are: (1) The topic words “bacterium,” “fungus or fungi,” “archaea,” or similar terms do not appear in the titles, abstracts, or keywords of the literature. (2) The main subject of the research isn't soil bacteria, fungi, or archaea. (3) The objects of the research aren't plants and/or soil. (4) The study does not primarily focus on the impact mechanisms of important soil microbial groups such as bacteria, fungi, and archaea on plants and soil.

Two hundred and forty articles were obtained according to the inclusion and exclusion criteria in total. Table 1 illustrates the publications and amounts related to the effects of bacteria, fungus, and archaea on plants and soil ( n = 161, n = 64, and n = 15, respectively). To gather secondary literature for citation references as publications within the scope of this study, a snowballing strategy was also employed, which involves reading pertinent papers in depth ( Greenhalgh and Peacock, 2005 ). By employing this technique, 26 more articles were found, bringing the total number of articles to 266. After that, the different kinds and amounts of literature were tallied: 234 journal articles, 24 conference papers, seven book chapters, and one short survey.

2.4 Integration and synthesis

In this step, a thorough analysis of the thematic content and research methods of the 266 articles was conducted. The content of the literature was then organized and classified, and the relevant research progress was analyzed and summarized. This stage reviewed existing achievements to help us clarify the specific mechanisms by which important microbial communities affect plants and soil, thus laying a theoretical foundation for their better application in promoting ecosystem functionality.

2.5 Results and reports

This stage is the last step of the entire process. After completing the aforementioned steps 1–4, we will present the final results and conclusions in this stage. An analysis of the 266 articles obtained during the retrieval phase will be conducted to determine the annual distribution of publications, global contributions by country, and research trends for the thematic keywords. The research findings will be detailed in Sections 3.1–3.3 of this study. Sections 3.4 and 3.5 will provide a summary of the mechanisms by which important microbial communities affect plants and soil, as well as their current application in ecosystem functionality.

3 Research results

3.1 publications and annual distribution.

The 266 pieces of literature that were selected were statistically analyzed, and the results are shown in Figure 3 . This shows how publications about the effects of large microbial populations on plants and soil changed around the world from 1992 to 2022. Based on the graph, it can be observed that the number of articles on this topic is generally increasing and is likely to continue to grow. Since the 1990's, global research on soil microbiology has undergone rapid changes, driven by technological advancements and increased attention to soil microorganisms. The protection and sustainable use of soil microbial diversity were formally proposed in 2002 at the 6th Conference of the Parties (COP6) of the International Convention on Biological Diversity ( Chandra and Idrisova, 2011 ; Zhu et al., 2017 ). This proposal raised awareness of the significance of soil microbiology in environmental science and agriculture. Afterwards, more research is being done on soil microbiology because more new microbial species are being found and molecular techniques for studying soil microbiota are getting better ( Keller and Zengler, 2004 ; van Elsas and Boersma, 2011 ), and research on soil microbes has entered a growth phase. In 2016, the United States launched the “National Microbiome Initiative,” aimed at exploring the functions of microbiomes in the environment and providing insights and solutions to major issues facing the twenty-first century, such as agriculture, energy, climate, and the environment. In the same year, advancements in high-throughput sequencing technology and bioinformatics tools made it possible to conduct more comprehensive research on the diversity and functionality of soil microbes ( Ciancio et al., 2016 ; Esposito et al., 2016 ). This has expanded the application field of microbiology and propelled it into a period of rapid development.

Figure 3 . Distribution of global publications on the impacts of bacteria, fungi, and archaea on both plants and soil from 1992 to 2022.

In terms of research content branches, there is a significant overall increase in the study of bacteria and fungi related to their impact on plants and soil. In contrast, the number of research papers on the impact of archaea on plants and soil shows slower growth. This phenomenon may be due to our limited understanding of archaea at present, as research in this area is still in the exploratory stage, resulting in a slow growth in the number of relevant research papers ( Baker et al., 2020 ). Additionally, the National Microbiome Initiative implemented in the United States has promoted the development of microbiology research ( Alivisatos et al., 2015 ), and the publication volume related to this topic has shown even faster growth since 2016.

3.2 Global distribution of contributors

An analysis of national affiliations was conducted on 266 articles, and the results are shown in Figure 4 . In terms of the global geographical distribution of research on the impacts of important microbial groups on plants and soil, China shows the highest level of research focus, with a frequency of 172 occurrences, making it the most prominent country in this field. The United States ranks second with 62 occurrences. India comes in third with a score of 41, ahead of Germany, France, and the Netherlands, all of which have scores above 20. This result reflects the high importance that China, as an agricultural powerhouse, places on research on soil microorganisms and the significant increase in attention given to soil and ecosystem conservation since 2016 ( Wen et al., 2022 ).

Figure 4 . Global distribution of countries contributing to research. The darker the color, the more articles contributed, and vice versa.

3.3 Research focus and trends

Visualize the main terms relevant to the topic from the selected 266 pieces of literature, and topic words and trends can be seen in Figure 5 . The keywords and their frequencies are detailed in Table 2 . The charts show that from 2002 to 2016, most of the research on how soil microbial communities affect plants and soil was focused on three words: “rhizobium,” “gene expression regulation,” and “inoculation.” All three of these words were used more than 20 times. Following closely were “fusarium,” “genes,” and “antibiosis,” with frequencies exceeding 15. After 2016, research focused on “(microbial) metabolism” with frequencies of 50, which is also the main research content of this article.

Figure 5 . Topic term research trend chart. The larger the node, the higher frequent occurrence the keyword.

Table 2 . Topic keywords and frequency.

3.4 Important soil microbiota's effects on plants and soils

3.4.1 the mechanisms of soil bacterial impact on plants and soil.

The most prevalent microbial population in soil, soil bacteria, have a major influence on plant health, soil texture, the nutrient cycle, and biodiversity. As seen in Figure 6 , they primarily affect soil structure and plant growth in the following ways: (1) By secreting enzymes that convert organic materials into nutrients that plants may use, bacteria take part in the breakdown of organic matter in the soil. For example, different proteases and polysaccharide hydrolases can be released by Bacillus subtilis and Bacillus licheniformis ( Bais et al., 2004 ; Lee et al., 2022 ). Cellulomonas uda is a wet cellulose bacterium belonging to the Cellulomonas genus that can break down complex macromolecular organic materials into simpler compounds ( Pérez-Avalos et al., 2008 ; Ontañon et al., 2021 ). Bacteria are also frequently employed to break down organic contaminants in soil. As an instance, phenolic compounds can be broken down by the Flavobacterium genus ( Larsbrink et al., 2016 ), and polycyclic aromatic hydrocarbons (PAHs) can be broken down by the Bacillus and Pseudomonas genera ( Saeed et al., 2022 ). These soil bacteria's decomposition processes contribute to the soil ecosystem's balance, soil environment purification, and soil quality improvement. (2) Soil bacteria aid in the cycling of nutrients, such as potassium, phosphorus, and nitrogen. Bacillus mucilaginosus Krass , for instance, has the ability to break down feldspar and convert potassium components that are insoluble into soluble nutrients ( Ahmad et al., 2016 ; Tallapragada and Matthew, 2021 ). Some types of soil bacteria, including those genera Erwinia ( Kim et al., 1998 ; Maldonado et al., 2020 ), Pseudomonas ( Alsohim et al., 2014 ; Oteino et al., 2015 ; Hamdali et al., 2021 ), Agrobacterium ( Belimov et al., 1999 ; Yu et al., 2011 ; Zheleznyakov et al., 2022 ), and Bacillus ( Ramírez and Kloepper, 2010 ), can change insoluble phosphate into soluble phosphorus that plants can use, which helps plants grow. (3) Beneficial bacteria can form symbiotic relationships with plants, providing nutrients to promote their growth and development. For example, Rhizobium forms symbioses with legume plants, while phosphorus-solubilizing microorganisms form symbioses with plant roots. They change nitrogen and phosphorus that plants can't directly absorb into forms that plants can use for growth through nitrogen fixation ( Chabot et al., 1996 ; Chebotar et al., 2001 ; Tokala et al., 2002 ; Tilak et al., 2006 ; Figueiredo et al., 2008 ) and phosphorus solubilization actions ( Chabot et al., 1993 ; Purnomo et al., 2005 ; Badawi, 2010 ; Turan et al., 2012 ; Berde et al., 2021 ; Lelapalli et al., 2021 ), respectively. (4) Soil bacteria can also suppress pathogens and reduce the occurrence of plant diseases. For example, Streptomyces ( Bressan and Figueiredo, 2008 ; Cordovez et al., 2015 ; Guo et al., 2020 ) and Bacillus subtilis ( Li et al., 2013 ; Sidorova et al., 2018 ) can produce antibiotics to inhibit pathogens. Bacillus can make plants produce antimicrobial peptides, antioxidants, and other chemicals that help plants defend themselves against pathogens and stop their growth ( Chakraborty et al., 2006 ; Ren et al., 2012 ; Revilla-Guarinos et al., 2014 ; Ahmed et al., 2022 ). Rhizobium can make secondary metabolites that can affect plant signaling systems in a direct or indirect way. This can cause plants to make good bacteria and become resistant to disease ( Zehnder et al., 2001 ). Plants can protect themselves and get induced systemic resistance from molecules made by Pseudomonas bacteria. These molecules include salicylic acid and volatile organic compounds ( Bigirimana and Höfte, 2002 ; Pieterse et al., 2003 ; Siddiqui and Shaukat, 2003 ). Bacillus ( Huang et al., 2014 , 2015 ) and Pseudomonas ( Karnwal, 2011 ) bacteria have been widely used in biocontrol and are known as biocontrol bacteria. (5) Some soil bacteria can produce plant hormones such as indole acetic acid (IAA), gibberellins, and cytokinins. They promote plant growth by influencing physiological processes such as plant growth, flowering, and fruit development. Streptomyces and Micromonospora can produce gibberellins ( Bottini et al., 2004 ), and PGPR-like Bacillus can produce IAA ( Pishchik et al., 2002 ; Zou et al., 2010 ). Research has shown that Bacillus subtilis WW1211 can promote plant growth by regulating coenzyme biosynthesis and redistribution ( Wang et al., 2021b ). (6) Soil bacteria can also improve soil structure and texture. Bacteria like Pseudomonas aeruginosa, Bacillus subtilis , and Chromobacterium violaceum can work together to make polysaccharide adhesives that stick to soil particles, help soil aggregates form, improve soil structure, make soil more aerate and hold on to water, and make soil more fertile ( Martens and Frankenberger, 1992 ). In fact, a single soil bacterium can perform multiple functional roles simultaneously. Some examples are Pseudomonas , which can help with nutrient cycling and plant defense mechanisms for biocontrol while also helping plants grow ( Jacobson et al., 1994 ); Streptomyces , which can make antibiotics to stop pathogens; and they produce gibberellins, which can help plants grow ( Solá et al., 2021 ). To sum up, soil bacteria play a collective role in promoting plant growth, shaping soil structure, and participating in ecological processes. Their interactions with plants and other microorganisms have significant impacts on plant and soil ecosystems. The interplay of these mechanisms collectively maintains the balance and stability of ecosystems.

Figure 6 . Conceptual map of mechanisms of action of important soil microbial communities.

External environmental factors frequently drive the mechanisms by which soil bacteria affect plants and soil. People often use environmental factors to induce bacteria to promote certain ecological processes in order to enhance ecosystem functionality. For example, the cadmium-induced rapeseed Xanthomonas campestris has shown adaptive resistance and cross-resistance to Zn. This characteristic can alleviate the toxicity of Zn in soil to plants, reduce the harm of metal to plants, and maintain the functionality of soil ecosystems ( Banjerdkij et al., 2003 ). In addition, adding bioorganic fertilizers can stimulate the quantity of Pseudomonas in the soil. Pseudomonas can play a role in biological control and enhance the suppression ability of plant diseases ( Tao et al., 2020 ). Another example is that biochar made from tobacco straw can alter the rhizosphere bacterial community, allowing growth-promoting bacteria to flourish and promote cherry growth ( Yang et al., 2022 ). However, remarkably, adding organic fertilizers can also stimulate the growth of pathogenic bacteria ( Tariq et al., 2022 ). Thus, when artificially inducing certain ecological processes, it is important to have a thorough understanding of the types of bacteria involved. Deepening the understanding and research of these mechanisms of soil bacteria can help in artificially regulating ecological processes and applying them to agricultural production. This will better harness the role of bacteria in enhancing ecological functionality and promoting beneficial ecological functions and services for humans.

3.4.2 The mechanisms of soil fungi's impact on plants and soil

Although there are some similarities and distinctions, the methods by which soil fungi affect plants and soil are similar to those of bacteria. As seen in Figure 6 , they mostly have the following effects on soil and plants: (1) Fungi aid in the breakdown of soil organic matter by reducing complex organic molecules to simpler ones and hastening the organic matter's mineralization process. For example, Paecilomyces lilacinus 112 can secrete hydrolytic enzymes to break down organic matter ( Constantin et al., 2022 ). (2) Soil fungi can convert nutrients such as phosphorus and iron, which plants cannot directly obtain, into available chemical substances ( Cao et al., 2015 ). For example, the Penicillium genus can break down insoluble phosphates ( Whitelaw et al., 1999 ; Wakelin et al., 2006 ; Gómez-Muñoz et al., 2018 ; Sang et al., 2022 ), which boosts crop output. (3) In order to provide plants with nutrients and water, soil fungi establish a mycorrhizal symbiosis with plant roots and mycelial networks are responsible for achieving this symbiotic relationship. Fungi that require organic carbon from plants for energy, which is produced through photosynthesis, such as AMFs, are essential for ecological processes. With 80% of terrestrial plants, AMF can create an arbuscular mycorrhizal symbiosis and help plants grow, take up water and nutrients, and maintain healthy soils by regulating the cycling of nutrients ( Lee et al., 2014 ; Mansur et al., 2019 ). Through competing for nutrients and space, the mycorrhizal symbiosis between AMF and plant roots can make plants more resistant to disease, fight soil-borne pathogens, make plant communities better able to handle stress from outside sources, and improve plant growth and soil quality. This helps restore ecosystems that can take care of themselves ( Barea et al., 2011 ; Pickett et al., 2022 ). According to Bianciotto et al. (2018) , AMF is a crucial component of ecosystems, and its scarcity or absence can reduce the ecosystems' ability to function as efficiently as possible. Hence, using AMF can recover damaged mining ecosystems, which has been applied extensively ( Wang, 2017 ). (4) Some soil fungi can control diseases by producing antibiotics or metabolites harmful to plant pathogens. For example, the Trichoderma genus releases volatile organic compounds that activate the plant's defense mechanism against powdery mildew ( Yao et al., 2016 ; Lazazzara et al., 2021 ; Cabral-Miramontes et al., 2022 ). They protect plant health by competing for nutrients with pathogens or inhibiting their growth. In addition, some soil fungi mediate the production of signaling molecules to activate plant defense responses, induce the plant's immune system, enhance its resistance to pathogens, and reduce the occurrence of diseases. For instance, endophytic insect-pathogenic fungi can affect wheat growth through immune responses and related hormone signaling pathways ( González-Guzmán et al., 2022 ). Trichoderma atroviride is a fungus that lives inside strawberry trees and protects them from plant pathogens by releasing acids that break down cellulase, proteins, and other compounds that kill pathogens ( Martins et al., 2022 ). (5) Most soil rhizosphere fungi can secrete auxins, such as IAA, to promote plant growth. In the Takalar District, 17 different types of fungi were found to produce IAA. These included Trichoderma, Fusarium, Aspergillus, Penicillium , and Gliocladium ( Larekeng et al., 2019 ). Among them, Trichoderma produced the most IAA. In summary, soil fungi play a significant role in providing nutrients to plants, enhancing plant immune systems, decomposing organic matter, and improving soil structure. Through their interactions with plants and other soil microorganisms, they have important effects on plant and soil ecosystems. The synergistic effects of these mechanisms maintain the balance and stability of soil ecosystems.

However, the impact of soil fungi on plants and soil is not always beneficial. Research has shown that parts of fungi are a major source of pathogenic organisms. For example, Fusarium oxysporum , a devastating soil-borne pathogen, is a major source of plant diseases ( Rekah et al., 2001 ; Lisboa et al., 2015 ; Goncharov et al., 2022 ). Phytophthora capsici , a polyphagous plant pathogen, can infect dozens of plant species ( Saltos et al., 2022 ). Plectosphaerella melonis , a common pathogen of cucurbits in the United States and Spain, can infect plants such as gourds, peppers, tomatoes, basil, and parsley ( Tsekhmister and Kyslynska, 2022 ). As a result, it is essential to differentiate between beneficial fungi and pathogenic fungi when studying the mechanisms of fungal impact on plants and soil.

3.4.3 The mechanisms of soil archaeal impact on plants and soil

Compared to bacteria and fungi, our understanding of archaea is relatively limited, and current research still mainly focuses on isolating and identifying new archaeal taxa. However, existing research has shown that the roots and rhizosphere of plants create both anaerobic and aerobic microhabitats. These areas are perfect for MA and AOA organisms because they help release methane and are involved in the nitrogen cycle ( Bomberg and Timonen, 2009 ; Pump and Conrad, 2014 ; Moissl-Eichinger et al., 2018 ). Consequently, as seen in Figure 6 , soil archaea also affect plants and soil in different ways: (1) The decomposing activity of archaea can convert organic matter into nutrient substances that plants can absorb and use, promoting soil nutrient cycling. As an example, AOA can break down organic micropollutants in riverbanks, turning them into small molecules that help the cycle of nutrients and affect plant growth ( Zhao et al., 2022 ). (2) Archaea can facilitate the cycling of substances such as nitrogen, phosphorus, sulfur, and carbon, as well as produce Fe carriers, providing the required nutrients for plant growth. For example, AOA changes ammonia into nitrite during aerobic conditions. During anaerobic conditions, they work with nitrite-oxidizing bacteria to reduce nitrite to nitrogen dioxide during denitrification. This changes how nitrogen is changed on Earth, as seen in Figure 7 for the process ( Li et al., 2020 ; Kraft et al., 2022 ; Martens-Habbena and Qin, 2022 ). Archaea provide plants with the necessary nitrogen source through nitrification, promoting plant growth and development. Meanwhile, they also reduce nitrogen gas through denitrification, reducing nitrogen elements in ecosystems. These two processes interact with each other, regulating the nitrogen cycling process in ecosystems and maintaining ecosystem stability. For instance, the new DPANN archaea found in hydrothermal vents take in nitrogen and sulfur and break down amino acids and nucleic acids in the environment. This helps the cycling of nitrogen, phosphorus, and other elements ( Cai et al., 2021 ). MA, on the other hand, turns compounds into methane in the absence of oxygen, which helps the cycling of carbon in nature ( Evans et al., 2018 ). Moreover, halophilic archaea exhibit functional characteristics related to phosphorus and iron production, and they may promote plant growth by enhancing the ability of plants to uptake iron ( Dave et al., 2006 ; Al-Mailem et al., 2010 ). (3) Archaea can produce plant hormones such as IAA, which affect plant growth. For instance, Sulfolobus acidocaldarius , a type of thermophilic microorganism, has been seen to make 1,000 times more IAA than other plant extracts ( White, 1987 ). Additionally, metagenomic analysis of marsh vegetation provides evidence that archaea carry genes for synthesizing auxins, further supporting the notion that archaea can promote plant growth. (4) Archaea activate plant defense mechanisms in multiple ways, enhancing immune responses and protecting vegetation health. For example, archaeal volatile organic compounds (VOCs) can induce resistance in plants. According to research by Ryu et al. (2003) and Song et al. (2019) , Nitrosocosmicus oleophilus MY3 produces VOCs that aid in the growth of Arabidopsis thaliana . This suggests that the mechanisms by which archaea promote plant growth may be similar to those of PGPR. Also, archaea can talk to bacteria through intercellular signaling, and they do this by making chemicals like N-acyl-L-homoserine lactones, which affect plant growth and make plants defend themselves ( Gao et al., 2019 ). Besides, archaea can help plants adapt to abiotic stress environments. Some Euryarchaeota genus members, like Methanosarcina , can stabilize the harmful metal mercury (Hg), which means that plants can still grow even when heavy metals are present ( Ma et al., 2019 ; Zhang and Wang, 2020 ); Halophilic archaea can alleviate salinity, enhance crop tolerance, promote plant growth, and reduce the impact of environmental stress ( Naitam et al., 2023 ). In general, soil archaea have big effects on plant growth and soil quality. They do this by breaking down organic matter, nitrifying and denitrifying, making plant hormones, and starting up plant defense systems. They play crucial roles in soil ecosystems, maintaining soil health and plant-soil ecosystem stability.

Figure 7 . Diagram of ammonia-oxidizing archaea mechanism.

3.5 Application of the mechanisms by important microbial groups in restoring ecosystems

3.5.1 remediation of contaminated soils.

Microorganisms can be used to purify soil contaminated with heavy metals and organic compounds through their processes of dissolution and degradation, which improves soil quality and promotes ecosystem functionality to restore damaged ecosystems and a clean soil environment. For instance, functional bacterial strains can remediate contaminated soil. Researchers have found that the bacteria Paracoccus aminovorans HPD-2 and Azotobacter chroococcum HN can break down PAHs into smaller inorganic compounds, which lowers the amount of organic pollution in the soil ( Wang et al., 2023 ). Haloarchaea colonized in the rhizosphere of halophytes, in cooperation with other rhizobacteria, can effectively degrade crude oil. Utilizing these rhizosphere microbial mechanisms can remediate oil-contaminated, high-salt environments. Additionally, some good bacteria and fungi can help plants fight off pathogens, lessen the damage that metals do to plants, increase the production of hormones and enzymes that help plants grow, and work with plants to clean up polluted soils ( Ma et al., 2016 ). As an example, gene expression is changed by plant growth-promoting bacteria (PGPB) such as Bacillus paranthracis NT1 and Bacillus megaterium NCT-2 to help Solanum nigrum L . get rid of cadmium (Cd), making Cd less harmful to plants and opening the door for PGPB and plants to work together to clean up heavy metal-contaminated soils ( Chi et al., 2022 ). AMF that lives in harmony with plants can stop uranium from moving through soils and plant roots, which stops uranium from moving from roots to shoots and effectively cleans up soils that are contaminated with this metal ( Fomina et al., 2020 ; Wang et al., 2022a ). Some types of Brevibacillus can make IAA, which, along with AMF, pulls Zn out of the growth medium. This makes Zn-contaminated soils less toxic and helps plants grow ( Vivas et al., 2006a , b ). Through various mechanisms, these microorganisms efficiently remediate contaminated soils in an environmentally friendly manner, greatly benefiting ecosystem functionality.

3.5.2 Biological fertilizers and microbial agents

In agricultural production, based on the principle of microorganisms promoting plant growth, beneficial microorganisms can be cultured in vitro and made into biological fertilizers and microbial agents, which can be applied to improve crop yield, promote local ecosystem functions when needed, and assist in the restoration and rebuilding of ecosystems. For instance, some beneficial microorganisms can be used to produce biological fertilizers: nitrogen-fixing bacteria can convert atmospheric N 2 into NH 3 that can be utilized by plants, reducing the use of nitrogen fertilizers in agriculture ( Shantharam and Mattoo, 1997 ; Urquiaga et al., 2012 ; Smercina et al., 2019 ; Hu et al., 2021 ); Bacillus subtilis N11 with strong plant colonization ability can effectively control banana wilt disease and serve as a new type of bio-organic fertilizer ( Zhang et al., 2011 ); Rhizobium and Pseudomonads genera can be used as natural biological fertilizers to promote plant growth ( Liu et al., 2020b ); Beneficial microorganisms can also be selected and cultured in vitro to produce functional strains that can be expanded for use as microbial agents. Microbes like the members of Bacillus genus and nitrogen-fixing bacteria can help woody cuttings stay alive and grow in nurseries. They can also help them avoid pathogens and encourage lateral branching ( Plugatar et al., 2019 ). Penicillium can break down phosphates, make gibberellins and plant growth regulators to help plants grow, and be used as a bio-inoculant to boost crop yield ( Nunes et al., 2022 ). Some non-pathogenic fungi can also be made from nanoparticles that can effectively control plant diseases, kill bacteria, and lower the toxic effects of metals in the soil. These nanoparticles are commonly used in agriculture as nano-insecticides, nano-fungicides, and nano-herbicides ( Alghuthaymi et al., 2015 ; Cao et al., 2020 ). However, as certain organic fertilizers can also promote the growth of pathogenic fungi, caution should be exercised when using biofertilizers ( Tariq et al., 2022 ). This method should only be employed properly and efficiently after a thorough understanding of the characteristics and processes of microorganisms.

3.5.3 Biopesticides and nitrogen-phosphorus removal

Utilizing the characteristics and mechanisms of microorganisms, we can produce pesticides and eliminate nitrogen and phosphorus, which not only maintains the health of vegetation and enhances vegetation productivity but also environmentally and economically purifies the environment, effectively maintaining the stability of the ecosystem. Some strains from the genera Pseudomonas, Bacillus , and Trichoderma have been good at controlling various fungal diseases to reduce the damage that pathogens cause to plants. For example, (a) Arabidopsis thaliana plants that are colonized with Pseudomonas fluorescens are protected from pathogen invasion ( Wang et al., 2022b ); (b) Bacillus amyloliquefaciens hmb33604 is resistant to potato late blight ( Fan et al., 2021 ); (c) the strain of Trichoderma genus LZ42 releases volatile organic compounds that inhibit tomato seedling wilt ( Rao et al., 2022 ). Additionally, the antagonistic interactions between different microbial species can also suppress the growth of pathogens. For example, the antagonistic effect between Streptomyces violaceus G10 and Fusarium oxysporum effectively controls plant diseases ( Getha and Vikineswary, 2002 ); the combination of Penicillium and Bacillus genera can antagonize the infection of Fusarium oxysporum in banana plants ( Win et al., 2021 ). These beneficial microorganisms can be formulated into biological pesticides for the biocontrol of plant diseases, reducing the likelihood of vegetation being infected by pathogens. In terms of eliminating nitrogen and phosphorus elements, microorganisms can reduce environmental pollution (such as water eutrophication) through degradation and participation in element cycling, which indirectly promotes plant growth and maintains the stability of the ecosystem. For instance, AOA nitrification and denitrification processes reduce wastewater nitrogen levels, which aids in the resolution of water pollution issues ( Yang et al., 2021 ). According to Kurt et al. (2017) , this technique has also been applied to clean industrial wastewater and encourage the sustainable use of water resources. However, due to their slow growth and sensitivity to their surroundings, AOA can only survive in environments with high levels of ammonia and nitrogen, which indicates that further investigation and study are required ( Hu et al., 2023 ). Under aerobic circumstances, biological phosphorus removal can be accomplished via the physical adsorption and precipitation of microorganisms ( Massoompour et al., 2022 ). Based on the above content, the principles of microorganisms provide us with an effective way to maintain ecosystem stability and environmental cleanliness.

4 Discussion

4.1 comparison and limitations.

In previous studies, in-depth reviews have been conducted separately on the relationship between soil microorganisms and plants, as well as soil microorganisms and soil ( dos Santos and Maranho, 2018 ; Etesami et al., 2021 ; He et al., 2022 ; Abdelsattar et al., 2023 ; Chaudhary et al., 2023 ; Poupin et al., 2023 ). Compared to other review literature, this study focuses on the impact of soil microorganisms on plants, soil, and plant-soil ecosystems rather than just on the individual study of plants and soil. We consider that microorganisms directly or indirectly affect soil or plants in the process of influencing plants or soil, and the relationship between the two is very close ( Jing et al., 2022 ). This study gives a thorough and organized overview of how microbial communities interact at the phylum and genus levels, and explains the connections between microorganisms, plants, and soil, and stresses how these results can be applied to improving ecosystem functions to fix broken ecosystems and clean up the environment. According to the current study, it was found that a large number of studies about soil microorganisms have investigated the specific effects of specific microbial groups on plants and soil ( Trivedi et al., 2021 ). In this study, we define bacteria, fungi, and archaea as important microbial communities and summarize and clarify the mechanisms by which these three groups influence plants and soil. The research progress on how microorganisms play a role in environmental restoration was illustrated to provide a theoretical reference for future studies in this field. In addition, this study only discusses the effects of important microbial communities such as bacteria, fungi, and archaea on plants and soil separately. The interactions between other microbial communities and the complex relationships between plants and microorganisms that affect plants and soil still need further summarization and deepening, e.g., cyanobacteria and PGPR working together can control nutrient management to boost biomass production and clean up polluted or inappropriate environments ( Prasanna et al., 2012 ). Again, different plant species, like clover, alfalfa, and grasses, affect the diversity and function of rhizobacterial communities, which in turn affects the rate at which PAHs break down to effectively reduce pollution problems in the environment ( Wang et al., 2021c ).

4.2 Prospect and deficiency

Currently, it has been proven that utilizing the mechanisms of microbial action in different fields to promote ecological functions is a scientific, clean, and efficient approach. Next, it is necessary for us to screen and identify more economically and environmentally friendly beneficial microorganisms, expand their applications, promote ecosystem functions, and maintain ecosystem stability. On the other hand, the improvement of molecular information technology remains the greatest challenge and opportunity in current microbial research, which not only helps in identifying and screening more new microbial populations but also aids in predicting the ecological functions of these new populations. Utilizing beneficial ecological functions is crucial for maintaining ecosystem stability and environmental cleanliness. For instance, through genetic engineering combined with other technologies, important genes with microbial functions can be inserted into plants, enabling them to better withstand environmental pressures and protect plant health ( Ding et al., 2021 ; Reboledo et al., 2021 ). Last but not least, it is significant to figure out how important microbial communities work to ecological functions, which will help us apply these communities to manage the environment and restore ecosystems.

Finally, we acknowledge that there are limitations to this study. Firstly, the definition of important soil microbial communities in this paper is based on their relatively high proportions in the soil, their importance in influencing plants and soil, and their high relevance to the research theme of promoting ecosystem functions. If other soil microbial communities that meet the above-defined criteria are discovered, it may change the scope of this study and require corresponding adjustments to the conclusions. These microbial communities would also serve as complementary, important soil microbial communities in this study. Additionally, there may be some subjectivity in the final selection of literature during the retrieval process, which could introduce certain biases in the conclusions. However, this does not affect the overall direction of development in this field.

5 Conclusions

Soil microorganisms play an important role in environmental remediation. Clarifying the mechanisms by which microorganisms influence plants and soil, as well as their applications in environmental remediation, helps us restore and rebuild ecosystems in a green and clean manner, maintaining ecological balance and stability. We used the SLR method to screen 266 articles in this field and reached the following conclusions: (1) Over the past 30 years, the volume and research focus of publications on this topic have shown different changes over time. Overall, there has been a growing trend in the number of publications over the years, but research on archaea has grown more slowly. Studies on the mechanisms of important microorganisms have touched upon microbial physiological activities, the identification of new microbial species, and functional applications. (2) In terms of specific mechanisms, bacteria and fungi promote plant colonization through decomposing soil organic matter, solubilizing elements such as nitrogen, phosphorus, and potassium, symbiosis with plants, secretion of phytohormones, and influencing soil structure, soil development, and plant growth. They also protect plant health by inhibiting pathogens and inducing plant resistance, enhancing the disturbance resistance of the plant-soil ecosystem. Soil archaea degrade low-molecular-weight organic compounds, participate in the cycling of nitrogen, phosphorus, and carbon, and produce plant hormones and antibiotics, promoting plant growth, protecting plant health, and effectively maintaining ecosystem stability. (3) In practical applications, the principles of important microbial communities can be used to remediate contaminated soil and water, reducing environmental pollution. In addition, selected functional microorganisms can be used to produce biofertilizers, biocontrol agents, and pesticides, promoting plant growth, protecting plant health, enhancing the functionality of the plant-soil ecosystem, maintaining ecosystem stability, and improving disturbance resistance. (4) Identifying new microbial species and predicting their functions remain the greatest opportunities and challenges in the current field of microbial environmental remediation. Future research on soil microorganisms should focus on natural environments, explore more beneficial microbial communities, pay particular attention to the coupling of biodiversity and ecological functions, promote the stable enhancement of ecosystem functions, help rebuild and restore damaged ecosystems, and ultimately achieve the goal of environmental remediation.

Author contributions

XW: Conceptualization, Methodology, Project administration, Software, Validation, Visualization, Writing – original draft. YC: Data curation, Funding acquisition, Resources, Supervision, Writing – review & editing. SS: Conceptualization, Data curation, Project administration, Resources, Supervision, Writing – original draft.

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This research was funded by the Guiding Fund Project of Government's Science and Technology [No. Qian Ke He Zhong Yin Di(2023)005], the Science and Technology Innovation Talent Team Building Project of Guizhou Province [Qiankehepingtairencai-CXTD (2023)010], and Academic Talent Plan of Guizhou Normal University [No. Qian Shi Xin Miao(2022)B31&(2021)B15].

Acknowledgments

We are grateful to the Natural Science Research Program of Guizhou Provincial Department of Education and the Academic Seeding Fund program of Guizhou Normal University for funding our program. In addition, we particularly thanks to the editors and reviewers for their patience in reading.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher's note

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Keywords: significant microbiota, effect mechanisms, ecosystem, functions, stability

Citation: Wang X, Chi Y and Song S (2024) Important soil microbiota's effects on plants and soils: a comprehensive 30-year systematic literature review. Front. Microbiol. 15:1347745. doi: 10.3389/fmicb.2024.1347745

Received: 01 December 2023; Accepted: 11 March 2024; Published: 25 March 2024.

Reviewed by:

Copyright © 2024 Wang, Chi and Song. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Yongkuan Chi, 201907002@gznu.edu.cn

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Article Contents

1. introduction, 2. systematic literature review: methods and materials, 3. results of the bibliographic analysis of data, 4. applications of nanotechnology in ev, 5. summary of key research findings, 6. conclusions, declaration of competing interest, acknowledgements, author contributions.

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A systematic review of nanotechnology for electric vehicles battery

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Pulkit Kumar, Harpreet Kaur Channi, Atul Babbar, Raman Kumar, Javed Khan Bhutto, T M Yunus Khan, Abhijit Bhowmik, Abdul Razak, Anteneh Wogasso Wodajo, A systematic review of nanotechnology for electric vehicles battery, International Journal of Low-Carbon Technologies , Volume 19, 2024, Pages 747–765, https://doi.org/10.1093/ijlct/ctae029

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Nanotechnology has increased electric vehicle (EV) battery production, efficiency and use. Nanotechnology is explored in this electric car battery illustration. Nanoscale materials and topologies research has increased battery energy density, charge time and cycle life. Nanotubes, graphene and metal oxides improve energy storage, flow and charging/discharge. Solid-state and lithium-air high-energy batteries are safer, more energy dense and more stable using nanoscale catalysts. Nanotechnology improves battery parts. Nanostructured fluids reduce lithium dendrite, improving batteries. Nanocoating electrodes may reduce damage and extend battery life. Nanotechnology benefits the planet. Nanomaterials allow battery parts to employ ordinary, safe materials instead of rare, harmful ones. Nanotechnology promotes battery recycling, reducing waste. Change does not influence stable, cost-effective or scalable items. Business opportunities for nanotechnology-based EV batteries need more research. High-performance, robust and environmentally friendly batteries might make electric cars more popular and transportation more sustainable with research and development. An outline of EV battery nanotechnology researchexamines the publication patterns, notable articles, collaborators and contributions. This issue was researched extensively, indicating interest. Research focuses on anode materials, energy storage and battery performance. A research landscape assessment demonstrates EV battery nanotechnology’s growth and future. A comprehensive literature review examined nanosensors in EVs. Our study provides a solid foundation for understanding the current state of research, identifying major trends and discovering nanotechnology breakthroughs in EV sensors by carefully reviewing, characterizing and rating important papers.

The future of nanotechnology with electric vehicles (EVs) is uncertain. Researchers and engineers use nano-manipulating materials to boost EVs’ speed, efficiency and longevity [ 1 ]. Nanotechnology makes coatings for EVs, battery technology, energy harvesting, sensors, catalysis and lightweight materials possible [ 2 ]. By enhancing energy storage, charging speed, component weight and durability, nanotechnology makes electric automobiles more efficient and practical, which could help circumvent transportation system limitations. A cleaner, greener and more sustainable transportation future will be ushered in as nanotechnology improves the capabilities of electric automobiles and increases their widespread acceptability. Nanotechnology’s quick growth has generated exciting potential for the EV revolution [ 3 ]. The use of nanotechnology can improve the performance, efficiency and longevity of EVs. Battery technology, lightweight materials, energy harvesting, sensors, catalysis and coatings are all examples of applications where scientists and engineers have taken advantage of nanoparticles’ unique capabilities [ 4 ]. The batteries used in EVs are improving because of nanotechnology [ 5 ].

Graphene and carbon nanotubes are two examples of nanoscale materials that improve energy storage, electrode surface area and electrode conductivity. The lightweight materials made possible by nanotechnology may also impact EV design [ 6 ]. Nanomaterial composites, including carbon nanotubes and nanofibers, have superior strength-to-weight ratios. Lightening the load improves the vehicle’s mileage, range and handling. EVs use nanotechnology for energy collection. Nanogenerators convert the kinetic energy produced by the car’s motion and vibrations into electrical power. Nanostructured solar panels on the vehicle’s surface capture sun energy for EV charging [ 7 ].

Nanotechnology allows for the development sophisticated sensors that keep tabs on and control the operation of EVs. The susceptible and accurate nanosensors can measure temperature, pressure and gas concentrations, increase safety, give better battery management and enhance vehicle performance. Catalysis and fuel cell technologies, which provide alternatives to EV batteries, rely heavily on nanotechnology [ 8 ]. Using nanocatalysts, fuel cells can convert energy more cleanly. Fuel cells are an option for EVs because of their longer ranges and shorter recharging times [ 9 ]. Intelligent coatings and self-repairing materials can be produced via nanotechnology for EVs. Scratches, corrosion and weathering are all thwarted by nanocoating, and the coating’s self-healing properties repair minor damage [ 10 ]. These coatings’ durability, hydrophobicity and UV resistance increase EVs’ aesthetics, longevity and sustainability. As progress in nanotechnology advances, the outlook for EVs improves. Nanotechnology can make transportation systems more effective, practical and environmentally friendly [ 11 ]. Figure 1 shows the few primary ways nanotechnology can be applied to EVs. Cleaner, greener transportation may lie in the hands of EVs that utilize nanoparticles and nanoscale engineering.

Nanotechnology in EVs.

To further explore how EV batteries might be improved, it is vital to provide a more comprehensive explanation of the specific roles played by nanotubes, graphene, metal oxides and nanoscale catalysts. Nanomaterials are crucial in enhancing different performance characteristics of EV batteries [ 12 ]. The remarkable conductivity features of nanotubes and graphene enable efficient electron transit, hence increasing the overall conductivity of the battery. Metal oxides enhance batteries’ energy storage capacity, improving their efficiency in storing and releasing energy during charge and discharge processes [ 13 ]. In addition, catalysts at the nanoscale operate as promoters to enhance the speed of reaction kinetics, optimizing the charge and discharge efficiency. Furthermore, integrating these nanomaterials improves EV batteries’ security and durability by addressing concerns regarding excessive heat generation [ 14 ] and establishing a more substantial structural foundation [ 15 ]. An in-depth investigation into the unique effects of each nanomaterial on energy storage [ 16 ], conductivity, charge/discharge processes, safety and stability issues offers a detailed comprehension of their combined influence on the advancement of EV battery technology [ 17 ].

Nanotechnology has become a major force in improving the performance and economy of batteries for EVs. Materials like nanotubes, graphene, metal oxides and nanoscale catalysts play key roles in this. Nanotubes and graphene are great for storing energy because they are highly conductive and have a lot of surface area [ 15 ]. This makes them better electrode materials for lithium-ion batteries, which are widely used in EVs. These materials make it easier for electrons and ions to move faster, which leads to more energy and faster charging and discharging. On the other hand, metal oxides are great alternatives to standard cathode materials because they have higher capacity and better cycling stability. Their nanoscale size makes charge transfer even more efficient, which lowers the energy lost during cycles [ 18 ]. Graphene is a single layer of carbon atoms grouped in a hexagonal lattice. It is very good at conducting electricity and is also very strong. It makes the conductivity better when added to battery electrodes, which speeds up the charging and discharging processes. Nanotubes, like carbon nanotubes, have a lot of surface area and can carry electricity well. This has helped make electrode materials that are light and work well. Metal oxides, such as manganese oxide and titanium oxide, have better electrochemical qualities [ 19 ]. This meets the need for materials in EV batteries that have a higher energy density and last longer between cycles.

Moreover, tiny catalysts have completely changed the electrochemical processes that happen inside batteries. These helpers, which are usually made of precious metals like platinum and palladium, make the processes that reduce and release oxygen easier in fuel cells and metal–air batteries. This makes them work better overall. Nanostructuring these catalysts also makes them more active, which means they can convert and store energy more efficiently. When it comes to safety, nanomaterials help make improved battery systems possible [ 20 ]. Nanoscale coatings and additives can make lithium-ion batteries safer by making them more stable at high temperatures, lowering the risk of burning, and stopping dendrites from forming. Nanomaterials also make things more stable, which means batteries last longer. This means they don’t need to be replaced as often, and throwing away batteries has less of an effect on the world [ 21 ]. According to the researchers, adding nanotubes, graphene, metal oxides and tiny catalysts to the design and make-up of EV batteries is a big step forward for making them safer, more stable and better at storing energy and conducting electricity. As scientists continue to study and improve these nanotechnology uses, the chances of making electric transportation more efficient and environmentally friendly grow [ 22 ].

1.1. Importance of nanosensors

Sanguesa et al. (2021) reviewed EV battery technological trends, charging techniques and future research challenges and prospects. They discussed the pros and cons of lead-acid, nickel-metal hydride, lithium-ion and lithium-sulfur EV batteries. They also discussed EV charging standards and modes such as AC and DC, slow and rapid, wireless and bidirectional. They discussed nanosensors’ high sensitivity, low power consumption, miniaturization, fabrication, integration and calibration concerns for battery management [ 23 ]. A literature overview on EV technology and its various uses, including smart grid, vehicle to grid, and vehicle to home, was presented. Energy efficiency, emissions reduction, grid stability, battery cost, range anxiety and charging infrastructure were only some of the topics covered. Several global EV programs and projects were highlighted, including Tesla, Nissan Leaf and Dubai Smart City [ 24 ]. A systematic literature review (SLR) on EV consumer acceptance was published by Han et al. [ 25 ]. They synthesized the methodology, ideas and variables from 57 peer-reviewed publications published between 2015 and 2022 on EV purchase, behavior and usage intentions. They found that attitudes, norms, perceived behavioral control, awareness, knowledge, personal values, emotions and social influence affect EV adoption. They also highlighted literature gaps and suggested additional research [ 25 ]. The authors of Basu et al. (2018) provided a synopsis of EVs and EV sensors. The advantages and disadvantages of the various varieties of EVs, including fuel cell EVs (FCEVs), battery EVs (BEVs) and hybrid EVs (HEVs), were discussed. Additionally, they deliberated on the evolving landscape of micro-electro-mechanical system- (MEMS) based miniaturization for sensors and devices utilized in diverse EV applications and the various types of sensors associated with battery and position monitoring [ 26 ].

1.2. Literature review

The low-carbon mobility transition relies on plug-in EVs, including battery and hybrid EVs. Daramy-Williams et al. (2019) found many user experience topics, including driving and travel behaviors, vehicle interactions and subjective factors. EVs have increased the utilization of electric batteries. Many studies have developed and improved battery cell voltage equalization methods by adding noteworthy features [ 27 ]. These cell equalization techniques were the subject of a thorough and methodical review (Das et al., 2020). This SLR focuses on recent efforts that have built energy management storage system (EMSs) for HEVs [ 28 ]. The study conducted by Torreglosa et al. (2020) aimed to provide a quantitative analysis of the chosen works. Notwithstanding advancements in driving range and recharge alternatives, these and additional market impediments persist, rendering the present market share of BEVs inconsequential [ 29 ]. In light of the advancement of hydrogen fuel cell stacks, an emerging powertrain architecture concept for N1 class-type cars was described (Castillo et al., 2020). The fuel cell extended range electric vehicles (FC-EREV)concept combines a battery-electric arrangement with a hydrogen-powered fuel cell stack that acts as a range extender. The lithium-ion battery in EVs can sustain an operational temperature range of 15°C to 35°C employing a battery thermal management system (BTMS) [ 30 ].

Tete et al. (2021) conducted a comprehensive review of experimental and numerical analyses of BTMS utilized in electric and hybrid vehicles. This review encompassed approaches such as air, liquid, phase change material, heat pipe and refrigeration cooling for battery cooling systems. EVs need high-energy batteries. The maximum capacity of lithium-air battery theory using graphene under optimal electron conduction conditions and the experimental maximum obtained by optimizing the structure geometry, examples of structural engineering using carbon fiber and carbon nanotubes in cathode fabrication to perform the reaction properly while providing space for lithium oxide placement, are examined [ 31 ]. Suryatna et al. (2022) described the battery’s mechanism and analyze its constituent parts. As a result of the growth of green logistics and the support of new energy car development policies both domestically and globally, logistics and distribution have embraced EVs as an alternative to traditional fuel vehicles [ 32 ]. Ye et al. (2022) examined the most recent breakthroughs in EV routing models and solution algorithms in logistics and distribution. Passenger vehicles contribute significantly to glasshouse gas (GHG) emissions; thus, precise and current estimates of the comparative emissions of the key types of alternative power trains are essential to support evidence-based policy recommendations [ 33 ]. A systematic review and harmonization of the most recent scientific literature on this subject was presented in Raugei (2022). The findings show that battery BEVs are the most promising option for decarbonizing the passenger vehicle fleet across all global regions studied, with the potential for −70% reductions in GHG emissions compared to conventional gasoline-powered internal combustion engine vehicles. EVs are becoming increasingly prevalent as many nations set net-zero carbon targets for the foreseeable future [ 34 ].

The status, characteristics and application scope of global lithium ion battery (LIB) safety standards and regulations were examined by Lai et al. (2022). The rational test standard upgrade is reviewed in light of recent EV and energy storage power plant fires. Direct drive offers improved systematic economy, more flexible wheel control and better passenger comfort by eliminating the gearbox and transmission [ 35 ]. Cai et al. (2022) reviewed vehicle direct-drive methods and contemporary electrical machine improvements for direct-drive propulsion systems. FCEVs for long-haul applications can reduce road freight CO 2 emissions until long-distance battery-electric mobility matures, depending on the hydrogen fuel source [ 36 ]. Pardhi et al. (2022) examined FCEV powertrain topologies for long-distance HD applications, as well as their operating constraints, cooling requirements, waste heat recovery methods, cutting-edge powertrain control, energy and thermal management strategies, as well as over-the-air route data-based predictive powertrain management with V2X connectivity. Batteries powering EVs provide a promising way to reduce pollution and uncertainty.

In contrast to battery degeneration’s scalability and temporal constraints, machine learning (ML) approaches offer a non-invasive, accurate and low-processing solution [ 37 ]. Sharma and Bora (2023) evaluated these problems objectively and comprehensively. EVs are becoming mainstream as more governments set near-term net-zero carbon ambitions [ 38 ]. Senol et al. (2023) conducted a comprehensive literature analysis in their study, which focused on integrating power networks and Li-ion battery technologies, particularly emphasizing their performance under unfavorable weather circumstances [ 39 ]. Increasing multiscale modeling and design for battery efficiency and safety management was reviewed by Kiran MD et al. (2024). This article shows how machine learning-based data analysis in battery research has advanced, setting the groundwork for cloud and digital battery management to produce trustworthy onboard applications [ 40 ]. Electron and ion transport affect the battery’s energy production under application conditions and how much energy can be used [ 41 ]. The transport mechanisms of ions and electrons for active materials, in addition to positive and negative composite electrodes, were examined by Quilty et al. (2023). Simultaneously, contemporary EVs exhibit a confident capacity to reduce fossil fuel consumption [ 42 ]. Gevorkov et al. (2023) reviewed and analyzed multiport converters’ key characteristics, topologies, pros and cons and applications. Operando characterization is not new, but techniques that can track commercial battery properties under realistic conditions have unlocked a trove of chemical, thermal and mechanical data that could revolutionize lithium-ion device development and use [ 43 ]. The innovative dual-ion battery that is built on aluminum and has a cathode made of three-dimensional graphene possesses high-energy density, low cost and the ability to charge and discharge faster than other batteries, making it an ideal choice for grid storage and personal gadgets by Zhang et al. (2016) [ 44 ]. A novel calcium-ion battery with a high discharge voltage and 95% capacity retention can function consistently at room temperature. This battery has the potential to serve as an alternative to lithium-ion batteries by Wang et al. (2018) [ 45 ]. For sodium-based energy storage applications, this study conducted by Mu et al. (2020) successfully synthesizes high-performance anodes with high-fraction active materials. These anodes achieve good rate capability and cycling stability [ 46 ]. Zhang et al. (2023) examined the difficulties encountered by power systems that heavily rely on inverter-based resources (IBR), specifically emphasizing the inadequate capacity to support voltage and frequency. The authors suggest reorganizing the virtual synchronous generator (VSG’s) control blocks by aligning the control blocks of a VSG with the control perspective of conventional synchronous generators. The reorganization above streamlines and clarifies the control pathway governing the system’s active and reactive power output, enabling easier virtual inertia and attenuation parameters to be adjusted. The article presents a compact signal model for IBR controlled by VSG, including voltage and current loops modules, instantaneous power calculation and an LC (inductors (L) and capacitors (C)) filter [ 47 ]. A current discrepancy that occurs during charge and discharging in high-temperature superconducting (HTS) non-insulation closed-loop coils may result in novel phenomena, such as a rapid decrease or increase in magnetic field at particular positions, and may influence operational current judgment by Lu et al. (2022) [ 48 ]. Wu et al. (2023) centers on the advancements in rare earth-barium-copper oxides superconducting tapes, with a specific emphasis on the nanocomposite EuBa2Cu3O7-δ superconducting films. These films hold significant importance in the context of high-field magnet applications. The scientists utilize a pulsed laser deposition methodology with an exceedingly rapid growth rate of up to 100 nm/s—two orders of magnitude higher than traditional approaches. This enables them to accomplish both rapid growth and a substantial capacity to transport field current [ 49 ]. Shen et al. (2024) have presented an innovative energy management approach that seeks to optimize the functionality of energy storage systems in EVs, specifically focusing on reducing the aging of lithium-ion batteries caused by high-frequency power requirements. Fuzzy logic control and ensemble empirical mode decomposition are incorporated into the proposed method. At the outset, the power demand of EVs is decomposed into intrinsic mode function components. Subsequently, permutation entropy is utilized to reconstruct each component into low- or high-frequency components [ 50 ]. A proposal was made by Zhao et al. (2023) for the preview-based human-like trajectory planning model (PHTPM), which is subsequently evaluated and assessed through comparative and generalizability tests. The findings indicate that the implementation of the driver preview feature empowers PHTPM to precisely emulate the attributes of proficient drivers during left turns while surpassing them during right turns [ 51 ]. A critical scenario search technique for intelligent vehicle testing that is based on the social cognitive optimization algorithm has been suggested, and the findings show that the proposed method has the potential to increase both the search efficiency, and the coverage of important scenarios was figured out by Zhu et al. (2023) [ 52 ]. This research presents a novel model that has the potential to improve our understanding of the behavior of lithium deposition and pave the way for the development of stable and secure lithium metal anodes through the utilization of bimetallic metal organic framework- (MOF) derived materials in the construction of three-dimensional frameworks studied by Wei et al. (2023) [ 53 ]. Based on the findings of the analysis conducted by Yue et al. (2023), it can be concluded that the stability of the road system in the presence of incident effects is directly connected to the severity of the event, the signal control strategy, the penetration rate and the spatial distribution of autonomous vehicles. In conclusion, simulation results are carried out to demonstrate the efficacy of our suggested event management policy in enhancing the rate of recovery and the stability of road networks [ 54 ]. In order to solve the issue in which the response quality is decreased as a result of factors such as parameter mismatch and disturbance, an adaptive disturbance observer-based improved super-twisting sliding mode control (ISTSMC-ADOB) has been developed. With the purpose of achieving adaptive compensation, avoiding the usage of high-gain feedback, and expanding the applicability of the conventional disturbance observer, an adaptive disturbance observer, also known as an ADOB, was intentionally constructed by Lu et al. (2022) [ 55 ]. Yu et al. (2023) has discussed the difficulties that arise when attempting to implement aqueous zinc-ion batteries in practical applications. The paper focuses on the unstable electrode/electrolyte interface that is related to inhomogeneous zinc deposition and side reactions. The solution that has been offered involves the utilization of L-carnitine (L-CN) as an effective addition for the purpose of stabilizing electrodes and extending the lifespan of batteries. When L-CN is present in minute quantities, it exhibits a remarkable synergy between quaternary ammonium cations, COO− anions and hydroxyl groups. This synergy can influence the electrochemical deposition/insertion of Zn 2+ and the activity of water molecules [ 56 ]. Hou et al. (2023) have centered on developing a self-powered, lightweight biomimetic mouse whisker sensor (BMWS) that draws inspiration from the extraordinarily perceptive whisker detection exhibited by mice. The BMWS, unlike its predecessors that attempted to imitate animal whiskers, surmounts drawbacks, including a cumbersome design, dependence on external power and restricted application scenarios. By utilizing the triboelectric effect and a meticulously engineered framework, the BMWS exhibits exceptional capabilities in detecting collisions and maintaining signal stability. Intelligent early warning, direction identification, hole width discrimination and real-time distance sensing are among the many duties in which it excels [ 57 ]. A novel mechanism and technology for directed energy deposition-arc in the production of large parts, referred to as alternating-arc-based additive manufacturing and enabled by a polarity-switching self-adaptive shunt, are presented Yan et al. (2023). The experimental outcomes demonstrate that the proposed system facilitates the alternating passage of current through the welding wire and the substrate, generating electronegative arcs at the anode with the wire and electropositive arcs at the cathode with the substrate. By modulating the arcs, decoupling control between thermal force and mass transfer is accomplished. The electropositive arc is responsible for cathode cleaning and molten pool temperature, whereas the electronegative arc regulates wire melting, particle size and temperature [ 58 ]. A battery–supercapacitor hybrid energy storage system and an accompanying energy management strategy are proposed in the article by Wang et al. (2024) in a response to the growing need for compact motors with high output torque, specifically for use in mobile robotics. This method draws inspiration from automobiles’ high-capacity hybrid energy systems. In conditions of minimal torque, the motor operates on battery power, with any excess power being directed toward charging the supercapacitor. When faced with torque overload conditions, the motor is supplied with high current by swiftly discharging the supercapacitor, which guarantees an instantaneous increase in output power. The objective of the energy management strategy is to regulate the supercapacitor’s charging and discharging processes to prevent interference with the motor’s power supply from the battery and maintain current stability and control during discharge [ 59 ].

Process of SLR.

Flow chart of selection of data for bibliometric analysis.

The resources utilized in this SLR comprise bibliometric data extracted from various sources, such as journal articles, conference papers, scholarly surveys, books, essays and review articles. These sources are analyzed to determine author affiliations, country collaboration maps, citation patterns, keywords plus and the frequency of author keywords. The present study employs a systematic procedure to select research publications about nanomaterials in EVs. The literature search involved comprehensive exploration using databases like Scopus. Key search terms included ‘nanotechnology in EV batteries,’ ‘electrode nanocoating’ and related variations. Potential biases in article selection were mitigated by employing rigorous inclusion criteria, prioritizing peer-reviewed studies and avoiding undue influence from commercial interests. Figure 2 shows the process of SLR.

The Scopus article research selections were restricted in the first step, as shown in Figure 3 and Table 1 . From 1991 to July 2023, this analysis uncovered 2361 publications concerning EVs, nanotechnology, nanomaterials and sustainable development. We have 2322 articles remaining after eliminating concise surveys, notes, erratum editorials and letters. After publication status-based exclusions of research papers, 2309 remained. The final subjects that were excluded from consideration were the following: astronomy and physics, pharmacology, toxicology, pharmaceutics, earth and planetary science, health profession, agriculture and biological science, neuroscience, immunology, microbiology, business and management, accounting, economics, econometrics and finance, arts and humanities and dentistry. One thousand five hundred forty products were remaining.

Bibliometric analysis is a method that quantifies scientific publications by examining citation patterns. Citations, according to a bibliometric study, reveal the impact, influence and connections of academic journals [ 60 ]. The bibliometric analysis theory posits that scientific knowledge is transmitted through scholarly publications, wherein the number and caliber of citations gauge the scientific significance of an article or publication it garners [ 61 ]. Bibliometric researchers utilize citation data to quantify author output, journal influence, collaboration patterns and research trends. According to this theory, more people cite influential articles [ 62 ].

Additionally, intellectual networks between authors, institutions and fields may be reflected in citations [ 63 , 64 ]. By quantifying these citation patterns, bibliometric analysis evaluates research output, influence and collaboration [ 63 ]. Bibliometric research is regulated by mathematical and statistical models, including co-citation analysis, citation counts, h-index and bibliographic coupling. Academics can quantify and contrast scholastic output, discern notable works and authors and monitor the evolution of specific publications or research domains through these techniques. Citation patterns can disclose the structure and dynamics of scientific knowledge, aiding policymakers, institutions and researchers in making informed decisions regarding funding allocation, research evaluation and scholarly communication, according to bibliometric analysis [ 65 ].

By examining the bibliography, the most cited journals are determined. Energy and environmental science, nanoenergy, ACS Applied Materials and Interfaces and the Journal of Materials Chemistry , all of which have contributed to the study of nanomaterials in EVs, are utilized in the proposed research. Numerous scholarly periodicals indicate that nanomaterials are an emerging trend for energy storage and lightweight materials to reduce the weight of EVs, as demonstrated by this review [ 66 ]. Bibliometric analysis is an expert methodology that assists researchers in assessing the advancement of various approaches through the statistical distribution of data about citation analysis, keywords, author affiliation and country of contribution. The top journals are shown in Table 2 .

Detailed analysis of nanomaterials in EV data is presented in this section. In recent decades, the quantity of publications and literary sources is evaluated in Sections 3.1 and 3.2. Additionally, the country production analysis is presented in Section 3.3. The thematic evaluation of the keywords in the publications is presented in Section 3.4. Additionally, the collaboration map of the top 15 nations, most cited countries, country production over time and source dynamics is provided in Section 3.5. Section 3.6 provides a comprehensive analysis of the findings regarding author keywords, most common terms, a tree map of keywords and word clouds. Section 3.7 presents three field plots depicting the relationships between author keywords, author countries and keywords plus.

3.1. Literature analysis of publications

Figure 4 illustrates the annual distribution of publications from 1991 to 2023. However, upon examining the graph, we have identified a substantial fluctuation in the number of publications on nanomaterials in EVs. The number of publications in this field was ‘0’ before 2003 but rose after 2005. However, between 2012 and 2021, there has been a significant increase in the number of publications devoted to investigations. The calculated annual growth rate was approximately 12.79%.

3.2. Institutions

A visual representation of the collaborations between the five most prestigious institutions worldwide and their publications over the last 30 years is presented in Figure 5 . As stated, five major institutions contribute to the Scopus Journal, with Nanyang Technological University ranking first with 945 publications and Beijing University ranking second with 586 publications. Simultaneously, the University of Science and Technology of China, the School of Material Science and Engineering and Tsinghua University ranked third, fourth and fifth in the Scopus database from 1991 to 2023, with net publications of 528, 475 and 409, respectively.

Main information of data taken from Scopus.

Top 15 journals in the research area under the study, 3.3. analysis of countries.

Diverse nations have contributed to academic journals concerning the application of nanotechnology to EVs. The cumulative number of publications originates from the 10 countries in Figure 6 . Based on the analysis, China holds the highest position regarding the total number of articles. Following that, India and the United States occupied the second and third positions, respectively, with 1584 and 480 publications, while China produced 2370 articles within the specified period.

3.4. Thematic evaluation of keywords

Based on the evaluation of keywords from 1991 to 2023, as shown in Figure 7 , several authors have repeatedly utilized a small number of keywords. The following terms were examined following the analysis: self-powered sensors, batteries, triboelectric nanogenerator, nanotechnology, renewable energy, lithium-ion batteries, lithium-ion batteries, electrode material and electrochemical performance.

Annual distribution of the publications.

Top 5 productive institutions.

Top 15 countries with the maximum number of publications.

Thematic evaluation of keywords.

3.5. Country collaboration

The collaborative efforts of numerous nations are illustrated in Figure 8 , and Figure 9 shows the cloud-based structure of the various country collaborations. Based on the bibliometric survey’s analysis, the following countries have been identified as having the greatest number of collaborations in the field of nanotechnology for EVs: China, India, the United States, South Korea, Germany, Japan, the United Kingdom, the Philippines, Singapore, France, Australia, Canada, Italy, Spain and Malaysia.

Country collaboration map.

Cloud-based structure of the various country collaborations.

3.6. Keywords

Each node in Figure 10a corresponds to a distinct keyword. We opted to include only the top 50 out of 1000 keywords. The prevailing research topics identified were ‘nanotechnology,’ ‘lithium,’ ‘electrodes,’ ‘electric batteries,’ ‘nanostructured materials’ and ‘energy efficiency.’ A cloud structure of keywords is depicted in Figure 10b , which resembles the analysis in Figure 10a ; however, the distinction between the two figures lies in the quantity of keywords. The authors spent the most time gathering the following 50 keywords for the cloud of keywords: ‘nanotechnology,’ ‘lithium-ion batteries,’ ‘lithium,’ ‘electrodes,’ ‘electric batteries’ and lithium compounds. In contrast, Figure 10c illustrates the tree map structure of keywords as they appear in the publications of various authors.

a. Keyword co-occurrence network. b Keywords cloud of top 50 keywords. c. Tree map of keywords.

3.7. Three-field plot

The three field plots between author keywords, author countries and keywords plus were depicted in Figure 11 . The preceding 20-author keywords, utilized by most authors in their publications, are displayed on the left side of the plot. The author countries to which they belong are then indicated in the middle. The top 20 countries were analyzed, with China, the United States, Korea and India ranking the highest. The concluding segment of the narrative presents keywords plus, which includes the top 20 keywords and those used by the author. Concerning the keywords, this plot is structured to examine the research field and the authors’ collaborative network.

Three field plots between author keywords, author countries and keywords plus.

3.8. Bibliometric analysis challenges

Researchers may encounter several problems and restrictions when using R-Studio Biblioshiny for bibliometric analysis. One big problem is that bibliographic records are very different from one another. This is because different sources may use different formats and terminologies, which makes it hard to combine and standardize data. It might also be hard to ensure the bibliometric study’s accuracy if the data’s quality and completeness vary from database to database [ 67 ]. Another problem is that scientific literature is always changing; new papers are always being added, and databases might not be updated very often, which could mean the results are incomplete or outdated. Also, full-text articles may not always be available or easily accessible, which could affect the study’s completion. Researchers may also have trouble developing relevant search queries because the buzzwords and the criteria they use to include or leave out information can greatly affect the results [ 68 ]. Even with these problems, R-Studio Biblioshiny for bibliometric analysis is still a useful tool that helps researchers get around many of these issues and learn important things about the scholarly world.

Additionally, picking the right bibliometric indicators and measures presents different problems. To measure the effect, output and teamwork, researchers must be meticulous in picking the right factors. If you misinterpret metrics or only use a small group of indicators, you might come to the wrong conclusions about the importance of certain study topics or authors [ 69 ]. Also, because some fields are changing and combining different areas of study, it might be hard to correctly group papers into the right research domains. When doing bibliometric analysis, it is also very important to think about ethics, especially when there are disagreements about who wrote a paper or when someone cites themselves in a paper. To ensure the results are accurate, it is important to balance automated data extraction and human verification [ 70 ]. There may also be problems with how bibliometric analyses can be repeated since different researchers may get different results depending on how they read parameters or how the data sources are changed. Even with these problems and restrictions, R-Studio Biblioshiny has an easy-to-use design and many tools that can help with many of them. As software is constantly updated and made better, some problems may be solved. This makes bibliometric analysis a useful and ever-evolving way to understand the scholarly scene. As the scientific literature changes quickly, researchers should stay alert and deal with these problems to ensure their results are strong and correct [ 71 ].

Nanotechnology is currently leading the way in transforming numerous aspects of EV technology. Nanoscale materials, including carbon nanotubes and graphene, are important in battery technology because they improve electrode conductivity, surface area and energy storage. This leads to enhanced energy density, prolonged battery life and accelerated charging, effectively mitigating apprehensions regarding charging durations and range anxiety [ 72 ]. In addition to batteries, nanomaterials can generate lightweight and robust materials for EV construction. This is demonstrated through the integration of carbon nanotubes and nanofibers into composites. This phenomenon fortifies the structure and positively impacts the vehicle’s energy efficiency, range and overall performance [ 73 ]. Energy harvesting in EVs is further facilitated by nanotechnology, as nanostructured solar panels capture solar radiation, and nanogenerators convert mechanical energy into electrical power, thereby contributing to sustainable electricity generation [ 74 ]. Incorporating nanomaterials into supercapacitors and energy storage improves regenerative braking and power delivery, increasing energy storage and charge–discharge rates [ 75 ]. Nanosensors, renowned for their accuracy and sensitivity, provide advantages to EV control and monitoring systems by facilitating the observation of gas, pressure, temperature and concentrations for enhanced performance and safety.

Furthermore, progress in catalysis and fuel cell technologies is facilitated by nanotechnology, which has the potential to supplant conventional EV batteries and enhance the efficiency of energy conversion [ 76 ]. Intelligent coatings and self-healing materials, enabled by nanotechnology, safeguard EV surfaces against corrosion, weathering and abrasion by forming hydrophobic, UV-resistant and durable coatings that increase longevity. Fundamentally, nanotechnology manifests as a paradigm-shifting influence, initiating an era of enhanced EV performance, sustainability and innovation [ 77 ]. Figure 12 shows nanotechnology’s applications, challenges and future perspectives in EVs.

Nanotechnology for EVs.

4.1. Challenges of nanotechnology in EVs

Although nanotechnology can greatly enhance EVs, it presents considerable obstacles that must be thoughtfully assessed and resolved inventively. The significant challenge of scalability arises from the difficulties associated with mass-producing nanotechnology processes, which hinder the consistent and superior integration of nanomaterials into EV components. The complexities inherent in nanotechnology further complicate issues about scalability, thereby demanding progress in manufacturing methodologies to guarantee extensive implementation [ 78 , 79 ]. Nanotechnology materials and processes are more expensive to manufacture; thus, cost-effectiveness is essential. Nanomaterials in EV batteries and lightweight components may raise production costs, affecting EV affordability and consumer acceptability. Another major challenge is EV nanoparticle and nanostructure stability. Nanomaterials must be durable and reliable to maintain EV performance and safety, especially under harsh operational conditions [ 80 ].

These stability difficulties require continual research and development to improve nanomaterial robustness and compatibility with EV dynamics. In EV nanotechnology integration, environmental concerns are essential [ 81 ]. Nanomaterial manufacturing, use and disposal must meet environmental goals to address health and ecological problems throughout the life cycle of nanotechnology in EVs. Research and collaboration between the scientific and manufacturing sectors aim to overcome these limitations and advance nanotechnology’s use in EVs. Scalability, cost-effectiveness, long-term stability and environmental impact can be addressed to realize nanotechnology’s transformative promise in EVs, enabling sustainable, efficient and widely accessible electric mobility [ 82–84 ].

4.2. Commercial potential of nanotechnology in EV batteries

The discourse on the commercial viability of nanotechnology-enabled EV batteries involves a comprehensive examination of the promising breakthroughs and the obstacles and limitations that hinder their widespread use. Nanotechnology has become a robust and influential factor in energy storage, specifically EV batteries [ 85 ]. These technological developments are crucial for achieving higher performance metrics, including increased energy storage capacity, enhanced conductivity, efficient charge and discharge cycles and enhanced safety and stability. When considering the economic potential of nanotechnology in EV batteries, it is crucial to carefully examine the obstacles that could impede its smooth absorption into the market [ 86 ].

An essential component of this topic revolves around comprehending the market’s preparedness for EV batteries that incorporate nanotechnology. Although the potential advantages are significant, the market must be sufficiently prepared to adopt these technical advancements. Market preparation covers various elements, including establishing infrastructure for manufacturing and distribution, creating regulatory frameworks that can handle emerging technology and promoting consumer awareness and acceptance [ 87 ]. Evaluating the present level of preparedness offers valuable information regarding the timeframe and viability of implementing nanotechnology-enhanced EV batteries on a broader scope. Simultaneously, it is crucial to tackle the obstacles posed by incorporating nanotechnology in EV batteries. The issues encompass a wide spectrum, from intricate technological obstacles to financial constraints. Nanomaterials, including nanotubes, graphene and metal oxides, provide remarkable advancements at the molecular scale. However, their ability to be scaled up and their cost-effectiveness in industrial applications must be thoroughly evaluated. The complexities of production processes, potential environmental consequences and the requirement for specialized knowledge provide problems that require careful and thorough consideration [ 88 ].

Moreover, the conversation should include the possible obstacles to the adoption that could hinder the universal approval of nanotechnology-enabled EV batteries. Barriers can appear in different ways, such as economic, technological and societal issues. Economic factors encompass the aggregate expenses of production, prospective price increases for consumers and the economic viability for producers [ 89 ]. Technological impediments encompass challenges such as the requirement for standardized testing protocols, concerns regarding reliability and compatibility issues with current infrastructure. The success of nanotechnology-enabled EV batteries is heavily influenced by societal variables, including how the public perceives and accepts new technologies. The economic feasibility of EV batteries enhanced by nanotechnology also depends on effectively addressing safety issues and environmental consequences [ 90 ]. Although nanomaterials provide advancements in battery safety and stability, it is crucial to conduct a comprehensive assessment of the potential hazards related to the manufacturing and disposal of nanomaterials. Establishing or modifying regulatory frameworks is necessary to guarantee the secure and conscientious incorporation of nanotechnology into the electric car industry [ 91 ].

Strong competition in the EV sector complicates commercializing nanotechnology-enabled batteries. Given the ongoing advancements in traditional battery technologies and other energy storage solutions, it is crucial to consider the placement of nanotech-enabled batteries in this changing landscape [ 92 ]. An essential aspect of evaluating the commercial success of nanotechnology-enabled EV batteries is comprehending the competitive dynamics, developing distinctive selling factors and distinguishing them from existing technologies. Moreover, the influence of government regulations and incentives in defining the commercial path of nanotechnology-enabled EV batteries should not be underestimated [ 93 ]. Implementing supportive regulations, financial incentives and research funding can greatly expedite the implementation and acceptance of these advanced energy storage options. On the other hand, if there are no well-defined norms or obstacles in regulations, it could impede innovation and discourage investment in the emerging market of nanotechnology-enabled EV batteries [ 94 ].

4.3. Nanoscale breakthroughs boost battery performance

One of the most important factors in improving the overall performance of EV batteries is the adoption of nanoscale enhancements, which are advances on a smaller scale. The essential components of the battery, such as electrodes and catalysts, are subjected to transformations due to using materials such as nanotubes, graphene, metal oxides and nanoscale catalysts [ 95 ]. The nanoscale dimensions of these materials contribute to an increase in surface area, an improvement in conductivity and an enhancement in electrochemical characteristics. Due to these enhancements, the battery’s cycle life is extended, its charge and discharge rates are accelerated and its energy density is increased [ 96 ]. Furthermore, the improvements in nanotechnology also address safety concerns by reducing the dangers of overheating and limiting the creation of dendrites. As a result, the cumulative effect of these small-scale advancements dramatically raises the overall efficiency, reliability and sustainability of EV batteries, paving the way for a more optimistic future in electric transportation [ 86 ].

4.4. Future perspectives of nanotechnology

A comprehensive evaluation of the environmental consequences of incorporating nanomaterials and nanotechnology processes into EVs is imperative. This underscores the criticality of assessing health and environmental risks to ascertain nanotechnology’s feasibility and ecological sustainability in this domain. When considering the future, nanotechnology can fundamentally transform numerous facets of EV technology [ 97 ]. Nanomaterials possessing enhanced energy storage properties such as higher energy densities, faster charging capabilities and longer cycle lifetimes hold promise for substantially augmenting EV batteries’ range, efficiency and overall performance. Furthermore, continuous investigations into nanomaterials have the potential to yield sophisticated lightweight materials that integrate composites and nanoscale structures. Such materials would augment EV construction materials’ strength while decreasing their weight. Consequently, this could enhance maneuverability, range and energy efficiency [ 98 , 99 ].

Nanotechnology can create smart, functional surfaces with self-cleaning, anti-fogging and anti-icing qualities that improve EV visibility and efficiency while reducing maintenance. Nanosensors in EVs enable real-time battery health, temperature and performance monitoring, improving charging, safety and maintenance [ 100 ]. As research advances, sustainable nanomaterials that use abundant, non-toxic resources to make the EV sector more environmentally friendly become a priority. As EV nanomaterial recycling reduces resource depletion and waste throughout the lifecycle of EVs, it supports a circular economy. These advances demonstrate the continuous commitment to leverage nanotechnology’s transformative potential in EVs, balancing innovation with environmental responsibility for a sustainable electric mobility future [ 101 , 102 ].

Readers interested in delving more into nanotechnology in batteries will find that internet resources that clarify complicated topics are an excellent place to begin their exploration [ 103 ]. Websites like Nano.gov provide explanations of nanotechnology and its uses that are suitable for beginners. These explanations include the role that nanotechnology plays in battery technology. In addition, educational sites such as Khan Academy offer illuminating video lessons on nanoscience and its applications in the real world, which makes the process of learning more interesting and accessible [ 104 ]. Exploring articles on respected science websites such as Scientific American or National Geographic can provide in-depth yet understandable insights into how nanomaterials contribute to breakthroughs in battery efficiency. This is especially helpful for individuals who prefer a more hands-on approach [ 105 ]. The last point is that books such as Nanotechnology for Dummies by Earl Boysen and Nancy Boysen offer a straightforward guide to comprehending nanotechnology. This makes it an invaluable resource for readers anxious to dispel the mystery surrounding this fascinating field [ 106 ].

This study conducted a systematic literature analysis to analyze bibliometric data from various sources to gain insights into the changing research landscape of nanomaterials in EVs. The study primarily examined author affiliations, nation collaboration maps, citation patterns, keywords and the frequency of author keywords. The research publication selection process is depicted in Figure 2 , highlighting the meticulousness and thoroughness of the literature evaluation. Between 1991 and July 2023, the research yielded 2361 articles about EVs, nanotechnology, nanomaterials and sustainable development. By applying the exclusion criteria, the selection was narrowed down to 1540 articles that were deemed relevant, resulting in a strong dataset for bibliometric analysis. Prominent journals such as Energy and Environmental Science , Nano Energy , ACS Applied Materials and Interfaces and the Journal of Materials Chemistry emphasized the importance of nanoparticles in the context of EVs. The significance of bibliometric analysis in comprehending the impact, influence and interconnections among academic journals was underscored. Citation patterns were employed to assess the scientific value of articles, monitor research trends and evaluate collaboration networks. The study demonstrated the ever-changing nature of how scientific knowledge is spread and highlighted the importance of bibliometric research in guiding politicians, institutions and researchers when making decisions. Figure 3 depicts how the Scopus article research selections were reduced in the first step.

Section 4 included an in-depth examination of nanomaterials in EVs, including a study of publishing trends, country production, thematic evaluation of keywords, collaboration maps and field plots. Figure 4 depicts the yearly dissemination of publications from 1991 to 2023. It shows a notable increase in research efforts beginning in 2005, with an estimated annual growth rate of around 12.79%. The collaboration map depicted in Figure 5 prominently showcased the significant contributions made by prominent universities, with Nanyang Technological University and Beijing University taking the lead in research endeavors. Figure 6 provides an overview of the worldwide contributions to nanotechnology in EVs, with China, India and the United States as the primary countries leading in this field. The analysis of keywords ( Figure 7 ) revealed the consistent presence of terms such as self-powered sensors, batteries, triboelectric nanogenerator, nanotechnology, renewable energy, lithium-ion batteries, electrode material and electrochemical performance. These terms indicate the main themes of the research. Figures 8 and 9 depict the cooperative endeavors of several nations, with particular emphasis on China, India, the United States, South Korea, Germany, Japan, the United Kingdom, the Philippines, Singapore, France, Australia, Canada, Italy, Spain and Malaysia, which are recognized as significant contributors in this sector. Figures 10 and 11 visually depict keyword analysis, revealing the most common study topics and the network of writers who collaborated. A list of the 50 most significant keywords was compiled, which included terms such as ‘nanotechnology,’ ‘lithium,’ ‘electrodes,’ ‘electric batteries,’ ‘nanostructured materials,’ and ‘energy efficiency.’ The treemap structure ( Figure 10c ) displayed the distribution of keywords in publications by different authors, providing a detailed comprehension of the research landscape. To summarize, this SLR thoroughly examines nanomaterials in EVs, utilizing bibliometric methodologies to uncover patterns in research, collaborations and important theme areas. The results emphasize the increasing significance of nanotechnology in the progression of EV technologies. Research gaps may exist in comprehending the precise applications of nanomaterials, tackling issues related to scalability and cost-efficiency, and investigating potential environmental consequences. Potential areas for future research could prioritize interdisciplinary investigations, technological advancements and sustainable methodologies to facilitate the incorporation of nanomaterials into EV technologies. The findings obtained from this work provide valuable contributions to the wider discussion on the convergence of nanotechnology and electric cars, offering guidance for future research efforts in this rapidly changing and developing subject.

In conclusion, the bibliometric analysis conducted on nanotechnology in EVs provides valuable insights into this field’s research landscape and trends. The study highlights the growing interest and research output dedicated to nanotechnology’s applications in electric cars, showcasing its significance and potential impact. By examining publication trends, influential articles, collaborating institutions and key contributors, the analysis helps identify the key areas of research focus and the leading contributors in the field. This information can guide future research directions, collaboration opportunities and resource allocation. The analysis also highlights the challenges and opportunities associated with nanotechnology in EVs. It reveals the need to address scalability, cost-effectiveness, long-term stability and environmental impact to realize the potential of nanotechnology in this domain fully.

Moreover, the bibliometric analysis is a foundation for further exploration and understanding of nanotechnology in EVs. It provides a basis for researchers, policymakers and industry stakeholders to assess the progress made, identify knowledge gaps and develop strategies to accelerate the development and adoption of nanotechnology in EVs. Overall, the bibliometric analysis of nanotechnology in EVs is a valuable tool that enhances our understanding of the research landscape, highlights emerging trends and offers insights into the future perspectives of this exciting field. It paves the way for further research and collaboration, ultimately contributing to advancing and implementing nanotechnology in EVs for a greener and more sustainable transportation future.

The authors declare that there is no conflict of interest in this work.

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University, Saudi Arabia, for funding this work through the Research Group Program under grant no. RGP 2/88/44.

Pulkit Kumar: Conceptualization, Methodology, Software, Writing – Original Draft, Data Curation, Writing – Review & Editing; Harpreet Kaur Channi: Conceptualization, Methodology, Supervision, Writing – Review & Editing, Formal analysis; Atul Babbar: Investigation, Supervision, Resources, Formal analysis; Raman Kumar: Investigation, Supervision, Conceptualization, Methodology, Formal analysis, Writing – Review & Editing; Javed Khan Bhutto: Formal analysis, Investigation, Supervision T.M. Yunus Khan: Investigation, Visualization; Abhijit Bhowmik: Formal analysis, Investigation, Supervision; Abdul Razak: Formal analysis, Investigation, Supervision; Anteneh Wogasso Wodajo: Formal analysis, Visualization, Supervision

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Business Process Management Journal

ISSN : 1463-7154

Article publication date: 25 March 2024

The aim of this paper is to identify the main challenges and limitations of current business process management (BPM) development directions noticed by researchers, as well as to define the areas of the main BPM paradigm shifts necessary for the BPM of tomorrow to meet the challenges posed by Industry 4.0 and the emerging Industry 5.0. This is extremely important from the perspective of eliminating the existing broadening gap between the considerations of academic researchers and the needs of business itself.

Design/methodology/approach

A systematic literature review was conducted on the basis of the resources of two digital databases: Web of Science (WoS) and SCOPUS. Based on the PRISMA protocol, the authors selected 29 papers published in the last decade that diagnosed the challenges and limitations of modern BPM and contained recommendations for its future development. The content of the articles was analyzed within four BPM core areas.

The authors of the selected articles most commonly point to the areas of organization (21 articles) and methods and information technology (IT) (22 articles) in the context of the challenges and limitations of current BPM and the directions of recommended future BPM development. This points to the prevalence among researchers of the perspective of Industry 4.0 – or focus on technological solutions and raising process efficiency, with the full exclusion or only the partial signalization of the influence of implementing new technologies on the stakeholders and in particular – employees, their roles and competencies – the key aspects of Industry 5.0.

Research limitations/implications

The proposal of BPM future development directions requires the extension of the BPM paradigm, taking into account its holistic nature, especially unpredictable, knowledge-intensive business processes requiring dynamic management, the need to integrate BPM with knowledge management (KM) and the requirements of Industry 5.0 in terms of organizational culture. The limitation is that the study is based on only two databases: WoS and SCOPUS and that the search has been narrowed down to publications in English only.

Practical implications

The proposal of BPM future development directions also requires the extension of the BPM paradigm, taking into account the specific challenges and limitations that managers encounter on a daily basis. The presented summaries of the challenges and limitations resulting from the literature review are accompanied by recommendations that are primarily dedicated to practitioners.

Social implications

The article indicates the area people and culture as one of the four core areas of BPM. It emphasizes the necessity to account to a greater degree for the influence of people, their knowledge, experience and engagement, as well as formal and informal communication, without which it is impossible to use the creativity, innovativeness and dynamism of the individual and the communities to create value in the course of business process execution.

Originality/value

To the authors' knowledge, this is the first systematic review of the literature on the limitations of modern BPM and its future in the context of Industry 4.0 and Industry 5.0.

• Business process management (BPM)
• Future of BPM
• Industry 4.0
• Industry 5.0
• Systematic literature review

Szelągowski, M. and Berniak-Woźny, J. (2024), "BPM challenges, limitations and future development directions – a systematic literature review", Business Process Management Journal , Vol. ahead-of-print No. ahead-of-print. https://doi.org/10.1108/BPMJ-06-2023-0419

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Active Student Participation in Whole-School Interventions in Secondary School. A Systematic Literature Review

• REVIEW ARTICLE
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• Published: 04 May 2023
• Volume 35 , article number  52 , ( 2023 )

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• Sara Berti   ORCID: orcid.org/0000-0001-5238-1077 1 ,
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This review presents a reasoned synthesis of whole-school interventions seeking to improve the overall school environment by fostering active student participation (ASP) in school activities and decision-making processes. The aims are to describe the selected programs, assess their methodological quality, and analyze the activities soliciting ASP. Among the 205 publications initially provided by the literature search in the academic databases PsycINFO and Education Research Complete, 22 reports met the inclusion criteria of presenting whole-school interventions that solicit ASP in secondary schools, and were thus included in the review. Such publications referred to 13 different whole-school programs, whose implemented activities were distinguished on a 5-point scale of ASP levels, ranging from Very high ASP , when students were involved in a decision-making role, to Very low ASP , when students were the passive recipients of content provided by adults. This review contributes to the literature by proposing an organizing structure based on different levels of ASP, which provides clarity and a common ground for future studies on student participation. Overall, the in-depth description of activities offers a framework to researchers and practitioners for planning interventions aimed at improving the learning environment and contributing meaningfully to the far-reaching goal of encouraging student participation in school life.

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Introduction

Research in educational psychology is consistent in showing that the quality of the school environment largely affects student well-being. Indeed, students’ experiences of a supportive school context have a significant impact on positive behaviors, such as academic achievement (Brand et al., 2008 ; Hoy, 2012 ) and good relationships among students and between students and staff (Cohen et al., 2009 ; Thapa et al., 2013 ). Conversely, the poor quality of the learning environment predicts negative outcomes, such as substance use (Weatherson et al., 2018 ) or bullying (Låftman et al., 2017 ).

In view of this, schools need to face the challenge of implementing interventions aimed at changing and improving the learning environment in the direction of promoting positive behaviors and reducing negative outcomes. One of the most promising directions in this regard is based on the adoption of whole-school approaches whose key features are the focus on overall school systems instead of on specific problems (Bonell et al., 2018 ).

The literature on whole-school interventions is broad (see, for example, Charlton et al., 2021 , for an extensive review on whole-school interventions focused on school climate), but it suffers from two major gaps. First, it relies primarily on programs applicable to elementary schools, while studies on high school populations are rarer, presumably because of the multiple challenges derived from the implementation of programs in such complex contexts (Estrapala et al., 2021 ; Vancel et al., 2016 ). Second, despite the importance generally attributed to the active involvement of students in the programs, to our knowledge, no previous reviews have specifically investigated the degree and the characteristics of student participation in such interventions. To address these limitations, in this article, we present a systematic literature review on whole-school interventions carried out in secondary schools and based on programs that envisage students’ active participation and involvement.

Whole-School Interventions for Improving the Learning Environment

In educational research, some reviews and meta-analyses (Charlton et al., 2021 ; Merrell et al., 2008 ; Voight & Nation, 2016 ) have critically synthesized and discussed studies on school interventions aimed at improving the learning environment. These programs have considered different outcomes of improvement, ranging from a general focus on school climate dimensions—e.g., relational aspects, institutional organization, and safety—to more specific aspects, such as bullying, violence, or substance use. However, the degree of effectiveness of such programs remains controversial. For example, a meta-analysis by Ttofi et al. ( 2008 ) indicated that school-based bullying prevention programs were able to bring about positive results, while another meta-analysis on the same topic (Merrel et al., 2008 ) concluded that evidence in this direction was only modest.

More positive results concerning the effectiveness of interventions were reported by Allen ( 2010 ) with reference to programs conducted by means of a whole-school approach. In her literature overview of studies designed to reduce bullying and victimization, the author concluded that whole-school interventions generally showed at least marginal evidence of improvement. Despite these encouraging findings, the studies conducted with a whole-school approach in secondary education contexts were rare. Among these, a well-established framework of whole-school interventions mostly implemented in middle schools is the School-Wide Positive Behavior Support program (for reviews, see Gage et al., 2018 ; Noltemeyer et al., 2019 ), which is a multi-tiered framework engaging students, school staff, and families for the delivery of evidence-based behavioral support aligned to students’ needs (Horner et al., 2004 ). By and large, the study results in this framework are again promising in suggesting a connection between such programs and school improvement, although the evidence is generally moderate and only regards a few of the considered outcome measures.

The mixed or weak results reported in the cited reviews solicit further exploration of the specific characteristics of whole-school interventions. In particular, a major limitation of the literature is the lack of an in-depth analysis of the types of activities proposed to students in each program, especially as far as their direct involvement is concerned. Given the importance attributed to student engagement in school life (Markham & Aveyard, 2003 ), this is a relevant area of inquiry that can inform researchers and practitioners willing to design and conduct whole-school interventions calling for students’ involvement.

Student Involvement in School Intervention

The importance of students’ involvement and participation finds a theoretical ground in the self-determination theory (see Ryan & Deci, 2017 ), according to which people who are self-determined perceive themselves as causal agents in life experiences, being proactive and engaged in the social environment. Studies examining such human disposition in adolescence supported the relevance of self-determination for quality of life and identity development (Griffin et al., 2017 ; Nota et al., 2011 ) and as a full mediator in the negative association between stress and school engagement (Raufelder et al., 2014 ).

In the light of these assumptions, educational and school psychologists have launched scientific and professional debates on the ways in which schools can implement favorable conditions for students to feel active and co-responsible for their educational and academic pathways (Carpenter & Pease, 2013 ; Helker & Wosnitza, 2016 ; Schweisfurth, 2015 ). These debates have reached consensus across-the-board on the recognition that school change and improvement are best fostered by intervention programs in which students are offered opportunities to get actively involved in school life (Baeten et al., 2016 ; Voight & Nation, 2016 ). For this goal to be achieved, all educational agencies are called upon to promote interventions capable of supporting activities that require student involvement and participation (Antoniou & Kyriakides, 2013 ).

The importance of students’ active participation in the school environment has also been confirmed by a substantial amount of literature investigating over time the association between high student involvement and positive learning environments. Mitchell ( 1967 ) reported that school climate is related to the extent of student participation and interaction during school life. Epstein and McPartland ( 1976 ) showed that student opportunities for school involvement were related to satisfactory outcomes. In a 1982 review published, Anderson claimed that “the type and extent of student interaction that is possible within a school appears to be a significant climate variable” (Anderson, 1982 ; p.401). A few years later, Power et al. ( 1989 ) described a program implemented in several contexts and characterized by high student involvement, whose results showed that a high rate of student participation led to their capacity to take on responsibility for building an effective learning environment and positive climate. More recent studies (Vieno et al., 2005 ) have confirmed that democratic school practices, such as student participation in decision-making processes, play a significant role in the development of a sense of community at individual, class, and school levels. The review by Thapa et al. ( 2013 ) confirmed the importance of student classroom participation as a variable affecting school climate and academic achievement.

On these theoretical and empirical grounds, providing space to student voices in decision-making and school change emerges as a powerful strategy for improving school environments and enforcing the success of programs (Mitra, 2004 ). The construct of student agency fits in well with this approach, as it refers to the students’ willingness and skill to act upon activities and circumstances in their school lives (Lipponen & Kumpulainen, 2011 ). Representing adolescents’ authentic, proactive, and transformative contributions to school life (Grazia et al., 2021 ), agency is fostered by school environments capable of soliciting and valorizing students’ active participation in educational practices and school decisions (Makitalo, 2016 ) and encouraging them to feel co-responsible with teachers and staff for their school lives (Mameli et al., 2019 ). The value of agency has been confirmed by research showing its positive associations with motivation and the fulfilment of basic psychological needs (Jang et al., 2012 ) as well as with the perception of supportive teaching (Matos et al., 2018 ).

Despite the agreement on student participation as a crucial feature for the success of programs capable of improving students’ school life, to our knowledge, previous literature reviews on school interventions have not focused specifically on the extent and way in which students are given a voice and are involved in the programs. In view of this, in the present work, we set out to search, in the existing literature, for interventions specifically based on activities in which students were not just the recipients of activities but rather took on an active and decision-making role. For our purposes, we use the notion of active student participation (ASP) to include the variety of ways in which students are given the opportunity to participate actively in school activities and decisions that will shape their own lives and those of their peers.

Review’s Aims

Previous reviews (Charlton et al., 2021 ; Estrapala et al., 2021 ; Voight & Nation, 2016 ) have provided extensive descriptions of whole-school interventions aimed at improving school environments or reducing school problems, suggesting their effectiveness. Moreover, a growing amount of literature has found that students’ active involvement in their school life is a crucial feature for improvement. Our general goal was to move forward by conducting an in-depth examination of existing whole-school interventions based on activities promoting ASP in secondary schools, by providing a reasoned synthesis of their characteristics and implementation. The choice to focus on secondary schools was driven by the evidence that this developmental stage has so far received less attention in whole-school intervention research (Estrapala et al., 2021 ; Vancel et al., 2016 ).

Given the large heterogeneity of existing intervention programs, both in terms of participants (specific subgroups vs general student population) and targets of improvement (specific abilities vs general school environment), it was essential to set clear boundaries for the study selection. As this was a novel undertaking, we chose to focus on whole-school interventions directed to the overall student population and aimed at improving the school climate as a whole. This allowed us to select a reasonably homogeneous sample of studies, with the confidence that future reviews will advance our knowledge by considering more specific fields and populations.

The review’s aims were (a) to describe the selected programs on the basis of their focus, country, duration, age of participants, and research design; (b) to assess the soundness of the research design and methodologies adopted in each study in order to provide evidence of the methodological quality of the selected programs; and (c) to differentiate among various levels of ASP in the program’s activities and, for each of these levels, to describe methods and activities carried out in the programs.

The present review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 updated statement (PRISMA 2020; Page et al., 2021 ). In line with the terminology proposed by the authors, in the following sections, we use the term study for every investigation that includes a well-defined group of participants and one or more interventions and outcomes, report for every document supplying information about a particular study (a single study might have multiple reports), and record for the title and/or abstract of a report indexed in a database. In addition, for the specific purposes of the present review, we use the term program when referring to an implemented whole-school intervention that has specific characteristics and is usually named, since more than one study may be conducted with the same program.

Eligibility Criteria

Studies were eligible for inclusion in the review if they were (i) written in English language; (ii) published in peer-reviewed academic journals; and (iii) aimed at assessing psychological effects of whole-school interventions that solicit ASP in secondary schools; thus, studies in which students were involved solely as recipients of activities delivered by adults were excluded. Moreover, in line with the review’s aims described above, studies were excluded from the review if they were (i) focused on specific subgroups of students (e.g., ethnical minorities or LGBTQ students); (ii) solely aimed at improving specific skills (e.g., literacy or mathematics); and (iii) solely focused on physical health (e.g., nutrition or physical activity).

Information Sources and Search Strategy

A literature search was conducted via EBSCO, including the academic databases PsycINFO and Education Research Complete, last consulted on April 9, 2022. The entered search terms were school-wide interventions OR whole-school interventions OR school-wide programs OR whole-school programs OR school-wide trainings OR whole-school trainings AND secondary school OR high school OR secondary education. By means of the software’s automated procedure, we searched these terms in the abstracts and filtered the results according to the first two inclusion criteria, selecting articles in English and published in peer-reviewed academic journals.

Selection and Data Collection Process

The records of each study were screened by two researchers, and the potentially relevant studies were further assessed for eligibility by three researchers, who read the full text independently. Moreover, some records relevant to the purposes of the research were identified through the references of the included documents ( forward snowballing ; Wohlin, 2014 ). Data from each included report were searched by two researchers, who worked independently to extrapolate the information relevant to the review, which were (a) the study characteristics; (b) the indicators of methodological quality; and (c) the program activities.

Detailed information about the selection process is provided in the PRISMA flow diagram (Fig.  1 ). The literature search provided 205 total records, and reduced to 169 after the automatic deduplication provided by EBSCO. After the application of our inclusion and exclusion criteria, 62 records were selected for full text reading. Of the 107 excluded records, 37 did not report interventions (e.g., they presented only school surveys), 25 were informative papers on initiatives and/or interventions without assessments, 15 focused only on academic skills attainment, 11 referred to primary schools, 10 focused on minorities, 6 were reviews of books or DVDs, and 3 only evaluated physical health as an outcome. After the full text reading of the 62 selected reports, 48 were excluded as they only discussed aspects related to implementation (e.g., feasibility or fidelity) without assessing the psychological effects of the intervention on students ( n  = 23) or did not solicit ASP during the intervention ( n  = 25). Thus, 14 reports were included in the sample. In addition, 10 reports were identified by sifting through the references of the selected documents ( forward snowballing ; Wohlin, 2014 ). After the full text reading, two of them were excluded as they did not assess the effects of the intervention. At the end of the selection process, the final sample of the present review included 22 reports, which referred to 16 studies and 13 programs.

PRISMA flow diagram

Study Characteristics

The main information about each study is reported in Table 1 . As for the focus of the interventions, three macro-areas were identified: (a) prevention of violence (nine studies and twelve reports), including programs for less bullying, cyberbullying, dating violence, sexual violence, and aggression; (b) promotion of mental health (five studies and seven reports), including programs for addressing depression and suicide risk and for promoting general psychological health; (c) promotion of positive emotional and relational school climate (two studies and three reports), including programs for enhancing school connectedness and school climate.

Within each macro-area, in Table 1 , the programs are listed following the alphabetical order of the program name. Out of the studies focused on preventing violence, three referred to unnamed anti-bullying programs, which in the present review were labeled Anti-bullying_1 , Anti-bullying_2 , and Anti-bullying_3 ; the other studies on the topic referred to an anti-cyberbullying program named Cyber Friendly School ; an anti-bullying program named Friendly School ; a bystander program aimed at preventing dating violence and sexual violence, named Green Dots ; a program aimed at preventing bullying and aggression, named Learning Together ; and an anti-bullying program named STAC , which stands for Stealing the show, Turning it over, Accompanying others, Coaching compassion. The studies focused on promoting mental health comprised a school research initiative aimed at preventing depression, named Beyond blue ; an intervention aimed at promoting mental health, named Gatehouse Project ; and a program to prevent suicide risk, named Sources of Strengths . Out of the studies focused on promoting positive emotional and relational school climate, one referred to the Restorative Justice program, aimed at promoting healthy and trusting relationships within the school, and the other referred to a program aimed at the promotion of a good school climate, named SEHER , which stands for Strengthening Evidence base on scHool-based intErventions for pRomoting adolescent health.

A large majority of the studies were conducted in the USA, some were carried out in Australia and the UK, and a few studies in India and China. The studies varied in duration, ranging from one to seven school years, and the number of schools involved in the intervention, ranging from 1 to 75. About half of the studies involved students from all grades while the other half was targeted only for some grades. Lastly, the reports varied in its research design: the majority conducted experimental group comparisons (EGC), but also other quantitative research designs (O) were present along with some qualitative designs (QUAL). To make text and tables more readable, the 13 included programs were renamed with a program ID consisting of the initials of the program name. Similarly, the 22 included reports were renamed with a report ID , consisting in the program ID followed by the surname of the first author and the publication year, all separated by underscores. Report ID and Program ID are reported in Table 1 .

Assessment of Methodological Quality

To assess each report’s methodological quality, we searched in the literature for a rigorous and comprehensive set of indicators and eventually decided to use as a reference the standards for evidence-based practices identified by the Council for Exceptional Children (CEC, 2014 ), which include indicators on setting and program description, fidelity, and reliability of outcome measures. Although the standards were originally recommended for the specific field of special education, they are considered appropriate to evaluate studies in all educational settings and were previously used by Charlton et al. ( 2021 ) in a systematic review studying the effects of school-wide interventions on school climate perceptions. Given that our aim was not to identify evidence-based practices but more generally to assess the methodological quality of the reports included in the review, some of the identified indicators were not applicable to our material. For this reason, among all the indicators described in the document (CEC, 2014), we selected those that provided a general overview of each report’s methodological quality. The selected indicators, their corresponding number in the CEC document, and a short description for each are reported in Table 2 .

In more detail, we applied a more extensive set of indicators to reports which fit the CEC definition of experimental group comparison design (EGC, as reported in Table 1 ), where participants were divided into two or more groups, both randomly and non-randomly, to test the effects of the interventions. For reports based on qualitative analyses and on quantitative analyses not consistent with the EGC design (QUAL and O, as reported in Table 1 ), we used a more limited set of indicators (indicators 1 to 6, as described in Table 2 ) and included a brief description of the research aims and methods. In the assessment of methodological quality, interrater reliability was achieved as three independent researchers read each report in detail, and the attribution of each indicator was discussed and agreed upon.

The assessment of methodological quality for the EGC reports is summarized in Table 3 . The findings show that most studies were strong in contextualizing the research, clearly describing the intervention program (either directly or with references to previous work) and conducting quality analyses. Weak points emerged to be related to the assessment of fidelity implementation (indicators 4 and 5 in Table 3 ), both with reference to adherence to the intervention program and to the dosage received by participants. Results for studies with qualitative analyses or quantitative analyses not EGC are reported in Table 4 . Like the ECG reports, most of these studies appeared strong in contextualizing the research and describing the intervention program, while fidelity of implementation received less attention (indicators 4 and 5 in Table 4 ).

Levels of Active Student Participation

As required by our inclusion criteria, all the selected programs were based on interventions that solicited ASP. However, from the careful analysis of the studies, we realized that the program activities promoted very different forms of ASP. Three independent researchers thus considered in detail each activity described in the programs and eventually agreed to score it on a 5-point scale (see Table 5 ), distinguishing among activities that solicit various levels of ASP. The scale partly followed the school participation scale of the HBSC questionnaire as defined by De Róiste et al. ( 2012 ). It ranged from Very high levels of ASP, attributed to activities in which students were given a fully decision-making role, to Very low levels of ASP, given to activities in which students were just the recipients of activities delivered by adults. In line with our inclusion criteria, in no programs, students were involved solely as recipients of activities delivered by adults (Very low ASP). Moreover, levels were not mutually exclusive, so that each program might include different levels of ASP.

In Table 5 , we report all the considered ASP levels, with the specifically related activities, and the coding of each program. It should be specified that the distribution of activities in the various levels was based on a qualitative accurate analysis of the role attributed to students and not on the number of students involved in each program’s activities, which varied to a great extent. In more detail, Very high ASP was attributed to interventions in which students were involved in processes with a direct organizational impact on school roles, curricula, and policies; this included two types of activities, i.e., student involvement in decision-making processes and the formation of school action teams comprising students. High ASP was recognized when students were still involved in organizational activities, but their role was limited to the implementation of activities and did not directly impact on school curricula and policies; it consisted of three activities, i.e., presentation of students’ works, leading of activities for peers, and leading of activities for adults. Moderate ASP was attributed when students were asked to express their viewpoints and opinions, without having a decision-making power, however; it comprised activities in which students were called to express their points of view on various school issues, either by the provision of platforms to share ideas, concerns, or suggestions, or by the organization of interactive school assemblies, or by their involvement in surveys based on data collection (e.g., by means of questionnaires) on specific aspects of their school life. Low ASP was attributed when the students’ activation was limited to a specific task required within a structured format designed by other people, including training for student leaders, interactive group activities, and individual activities. Finally, activities were coded as Very low ASP when students were involved as the passive recipients of contents provided by adults, through lecture-style lessons, viewing of videos, or distribution of didactic material. The activities provided for in each program are described at length in the following paragraphs, considering activities scored in every specific level of ASP.

For the sake of completeness, in Table 5 , we added a final column in which we indicated additional program activities that involved the staff. They comprised the formation of school action teams made up of adults, training, and the provision of materials for the staff. As the description of these activities goes beyond the scope of our investigation, we will not describe them in detail.

Very High ASP: Making School Rules

As can be seen in Table 5 , activities implying involvement of students in decision-making processes were identified in six programs. In AB3, during a school assembly, students were invited to develop a whole-school anti-bullying policy, while in later activities, they were asked to identify strategies to be implemented in the school to prevent bullying. In CFS, school staff and student leaders conducted whole-school activities helping students to review school policies to promote a positive use of technology. In FS, the intervention aimed to help the transition between primary and secondary school was co‐developed with students who had already made such a transition. In GP, the use of peer support and leadership was encouraged to increase opportunities and skills for students to participate in decision-making processes within the school; in addition, at a classroom level, rules were negotiated by teachers and students and displayed in each classroom. In RJ, during the first year of implementation, staff and students developed a plan for pathways of primary, secondary, and tertiary restorative interventions; in the following years, students’ leadership roles and collective decision-making activities increased, so that students themselves were able to advance whole-school initiatives and activities, to map out course goals and determine which projects they would embrace. Finally, in SE, some health policies were discussed with the principal, teachers, and students before being finalized in a school action team meeting and disseminated at whole-school level.

Activities consisting in the creation of school action teams (or school action groups) including students and teachers were identified in three programs (see Table 5 ). In AB2, a school action group with both students and staff was formed to define action plans and training for staff on restorative practices at whole-school level and to implement a new school curriculum focusing on social and emotional skills. In LT, a school action group comprising around six students and six staff was formed to lay down school policies and coordinate interventions, based on the feedback from the student data collection. In SE, a school health promotion committee, consisting of representatives from the school board, parents, teachers, and students, was formed to discuss issues submitted by the students and to plan the activities for the future years based on the feedback from the activities already carried out. In addition, a peer group of 10 and 15 students from each class discussed health topics and student concerns with adult facilitators, in order to develop an action plan and to help in organizing various activities, such as contests and school assemblies.

High ASP: Organizing School Events

Three types of activities were included in this level of ASP. As reported in Table 5 , five programs solicited the creation of different student artifacts . AB1 included a student-made video on bullying to be presented to all the students. In GP, student artifacts were presented to audiences such as parents, other students, teachers, and members of the community. In SS, student leaders made presentations for peers to share personal examples of using the strengths provided by the program. In RJ, students engaged in collaborative, interactive writing activities based on analytical reflection for the realization of a rubric co-developed by students. SE included the contribution of all the students, teachers, and the principal in the realization of works like write-ups, poems, pictures, or artwork, on specific topics for a monthly wall magazine publication. SE also envisaged contests among students, such as poster-making and essay writing, linked to the monthly topic of the wall magazine.

Activities regarding the organization of student-led activities for peers were found in four programs (see Table 5 ). In CFS, student leaders (four to six in each intervention school) conducted at least three important whole-school activities to promote students’ positive use of technology for raising students’ awareness of their rights and responsibilities online; they also provided cyberbullying prevention trainings for peers. In SS, student leaders (up to six in the school) conducted activities aimed at raising awareness of Sources of Strengths , generating conversations with other students, providing presentations about the strengths proposed by the program, and engaging peers to identify their own trusted adults. RJ included student-led restorative circles with students, workshops for students, and peer-to-peer mentorship on restorative practices. In SE, student leaders (between 10 and 15 in each class) conducted peer group meetings to discuss on relevant health topics.

In two programs, student-led activities for adults were organized (see Table 5 ). In CFS, student-led activities provided information to the teaching staff about the technologies used by students and cyberbullying prevention training given to parents. In RJ, circles and workshops on restorative practices were implemented by the students for the staff.

Moderate ASP: Expressing Personal Views

Three types of activities were included in this level of ASP. As can be seen in Table 5 , two programs provided platforms where students could express their personal views on various topics. In BB, students, families, and school staff were provided with platforms to share information and communication on mental health issues. In SE, platforms were used to raise concerns, make complaints, and give suggestions, either anonymously or by self-identifying, on the intervention topics.

Interactive assemblies for students to discuss on the main intervention topics were organized in three programs (see Table 5 ). AB1 provided a first interactive school assembly to discuss respect and bullying, and later assemblies at class level to further discuss the themes emerged during the whole-school assembly. Similarly, AB3 included a school assembly where students were encouraged to get involved in the development of a whole-school anti-bullying policy, followed by three lessons during which the class teacher facilitated a discussion in each class aimed to raise awareness about bullying and to think about school-based solutions. SE included group discussions for generating awareness about health issues, to be discussed during the school assemblies that took place four times a month.

In six programs, students were given a voice by data collections to be used in the process of school changes (see Table 5 ). AB1 included a bullying report form that students, in addition to staff and parents, filled in to report bullying incidents. AB3 provided feedback from student data collection during the school assembly as a basis for discussing whole-school anti-bullying strategies. In LT, annual reports on students’ needs, drawing from student surveys in relation to bullying, aggression, and school experiences, guided the action teams to define school policies and coordinate interventions. In BB, summaries of student and staff data on current school structures, policies, programs, and practices related to student well-being, collected annually, were used by the team to create an “action plan” for changes across the school, both at the classroom and whole-school level. In GP, the profile emerging from the student surveys on school environment guided school teams in the definition of priority areas and strategies within each school, both by coordinating existing health promotional work and introducing new strategies that met the needs of a specific school. In SE, the school action team planned school activities based on reports and discussions on issues presented by the students.

Low ASP: Trainings

A low level of ASP was identified in three types of activities. As can be seen in Table 5 , four programs included trainings for student peer leaders . CFS provided a 10-h training for peer leaders to lead whole-school activities on the positive use of technology. GD provided a 5-h bystander training for student leaders to recognize situations and behaviors that could lead to violence or abuse and to identify active bystander behaviors to be performed either individually or collectively to reduce the risk or effect of violence. ST provided a 90-min training session and two 15-min booster sessions on bullying, which included icebreaker exercises, hands-on activities, and role plays. SS provided a 4-h interactive training for peer leaders aimed at developing protective resources in themselves and encouraging peers to grow such resources as well.

Eight programs included interactive group activities (see Table 5 ). In AB3, students worked in small groups to identify the types of bullying in the school and to discuss strategies to prevent bullying, with the support of bullying scenarios with discussion questions. In CFS, interactive activities included problem-solving, quizzes, and case studies on the use of technology to prevent cyberbullying. LT included various interactive activities aimed at preventing violence, ranging from informal practice, for example, using “affective” statements to communicate feelings, to formal practices, for example hosting a restorative “circle” where participants were encouraged to express emotions and create emotional bonds after problematic or disruptive behavior. BB provided a range of interactive teaching methods, such as small-group exercises, role plays, and quizzes, for reflecting on mental health issues. GP provided activities as small group work and class discussion, by also implementing interactive teaching strategies, such as using questions to kindle discussions and emphasizing the importance to consider different perspectives on a topic, encourage challenges, and debate ideas. SS included peer-to-peer messages and activities wherein student leaders shared examples of strengths that have helped them to overcome personal challenges and invited their peers to participate in interactive tasks. RJ included many interactive practices, such as restorative circles, interactive writing activities, and peer-to-peer mentorship to broaden the impact of restorative practices. SE included monthly contests for students, such as elocution, debates, and quiz games.

Finally, in BB program, some individual activities , in addition to group tasks, were conducted (see Table 5 ). Such activities consisted of individual writings and self-reflection on specific topics, aimed at building or enhancing sense of self-worth, belonging, control, purpose, future, and humor, which were considered to protect against mental health problems.

Very Low ASP: Students as Recipients

Activities in which student’s role was overall that of the recipients of actions taken from adults were of three types. As can be seen in Table 5 , six programs included lecture-style lessons . AB1 included the speech by a nationally known speaker about respect and bullying and the presentation of the Social Support System to students by their English teachers. AB3 included the presentation of summary feedback from the pre-test questionnaires during a school assembly and three lessons, delivered by the class teachers, on school bullying. CFS included lessons led by class teachers, aimed at improving online social skills, focusing, in particular, on positive communication, resilience, self-management, conflict resolution, and social responsibility. GD included a 50-min persuasive lesson led by adults focused on violence victimization, perpetration, and prosocial behaviors. LT included adult-led lessons on social and emotional skills. SE organized workshops led by teachers or program facilitators on effective study skills, such as time management, learning style, note-taking, reading comprehension, memorization techniques, and concentration techniques.

Three programs included the viewing of videos during the implementation (see Table 5 ). AB1 and AB3 provided a video on school bullying for all the students. BB provided video or DVD materials on mental health issues.

Finally, three programs provided students with informative materials (see Table 5 ). AB1 provided a form with several responses for intervening against bullying, which offered alternatives to the traditional method of apportioning blame and punishing bullies. Similarly, AB3 provided a worksheet on possible responses to bullying. FS provided educational magazines on bullying issues. BB provided many materials, such as individual student workbooks, a review poster, master copies of resources for all activities, and homework worksheets.

The aim of this systematic review was to provide a reasoned synthesis of whole-school interventions in secondary school capable of improving the school environment by assigning an active role to students. The first result that warrants consideration regards the number of publications that met our eligibility criteria to select whole-school interventions based on activities soliciting ASP in secondary schools. Despite the wide interest of researchers on the topic of whole-school interventions in general (see Bonell et al., 2018 ; Charlton et al., 2021 ), our selection and data collection process eventually provided only 22 reports referring to 16 studies that fostered ASP during the intervention. This result calls for further work in the field. Based on the emphasis given by educational and political agendas about the importance of empowering students in their role as active participants in schools, first of all, and in societies, subsequently, research should not overlook the question of how to improve their participatory skills by involving them in school activities and decisions (Markham & Aveyard, 2003 ). More investment in this direction is needed to evaluate the consistency and efficacy of the existing programs, to eventually reach consensus on the intervention protocols that schools can implement to improve their learning environment. Results related to each of our specific aims will be discussed in the following paragraphs.

Characteristics of the Selected Programs

As for the first aim of the review, concerning the description of the identified literature, several reflections arise from our results. Considering the year of publication, we found growing interest by researchers in the field, as most reports were published in the last few years, i.e., from 2018 to 2021. This may be considered positive indication that research has identified student participation in school interventions as a crucial topic on which to invest for future works. As for the focus of the selected literature, most of the included studies concerned the reduction of violent behaviors, referring for the most part to bullying, while the promotion of a more general positive emotional and relational school climate is the less investigated topic. Notwithstanding the overall need to fill in this limitation of research, these results suggest that future studies should address the issue of how it is possible to create better school environments for students starting from their own involvement and decision-making roles. This is consistent with the direction indicated by Bonell et al. ( 2018 ), who upheld the importance of focusing more on overall school systems rather than on specific problems. The implementation of a larger number of programs fostering ASP in order to improve school climate and learning environments would thus be important to understand how to support students in dealing with the variety of non-specific problems that can arise during school life. Indeed, as confirmed by the literature, a positive school climate is related to higher academic achievement (Berkowitz et al., 2017 ; Kutsyuruba et al., 2015 ) and fewer problematic behaviors, violence (Reaves et al., 2018 ), and psychological malaise at school (Aldridge and McChesney, 2018 ). Finally, looking at the country of the selected program implementation, most of the studies were conducted in the USA, some in Australia and the UK, and a few in India and China. To our knowledge, with the exception of the two anti-bullying programs carried out in the UK and included in the current review, no other studies were conducted in European countries. With caution, as in many other countries researchers may have developed programs that could not be included in this review due to the inclusion criteria, we consider this as a gap in the literature that future work should fill, especially considering that school policies and organizations are very different between continents. In this regard, it would be interesting both to replicate existing programs and to develop revisited or new interventions specifically adapted to the context of the country’s school system, a work that would also fulfil the aim to increase the ecological validity of the proposed activities.

Methodological Quality of the Selected Reports

The second aim of the review was to assess the soundness of the research design and methodologies adopted in the selected studies. In this regard, we found that the considered reports were robust overall, as they met most of the considered indicators of methodological quality. In particular, most of the studies, based both on EGC or on other designs, described and contextualized the intervention and provided adequate analyses. Beyond the generally good methodological quality of the included studies, consistently with previous examinations of intervention programs (Charlton et al., 2021 ), we found a weakness concerning the fidelity of implementation, as this indicator was observed in only about half of the considered programs. Given that fidelity is a fundamental aspect for the evaluation of the intervention efficacy (O’Donnell, 2008 ), future studies should consider this important factor, by adding it to the evaluation of the programs for providing adequate monitoring tools that include qualitative and process indicators. Overall, however, the literature on the interventions meeting the criteria for our review, albeit limited, relies on methodologically sound grounds that allow us to draw some conclusions on programs and activities actively involving students and to offer suggestions for researchers and practitioners in the field.

Program Activities and Levels of ASP

As for the third and last aim of the review, i.e., concerning the analysis and description of ASP activities proposed in the various programs, our results offer material for an innovative way to look at the programs and points the way to future research in the field. In particular, some points should be highlighted. First, we were able to show that a variety of ASP activities can be used in interventions, from those requiring students to directly act on school programs and policies to those in which students are merely involved as recipients of contents delivered by school staff. From the careful and independently conducted analysis of all program activities, we were also able to grade such activities on a scale ranging from very high to very low levels of ASP. This may be a useful tool for researchers, as it advances a way to develop and organize interventions fostering different levels of ASP activities, to be selected on the basis of the research focus and aims. The effort to identify different levels of ASP also has the merit of introducing some degree of clarity and order in the great variability of program activities. While the importance of student involvement and participation was generally recognized in the literature (Baeten et al., 2016 ; Schweisfurth, 2015 ), our in-depth description shows that not all forms of participation are equal, and thus offers a tool to differentiate between them. This advances our understanding of the concept of student participation both on a theoretical and methodological level.

Beyond this general picture of ASP activities, our findings show that the interventions based on the highest levels of ASP are those aimed at generally improving the school environment, i.e., the Restorative Justice and SEHER programs. These programs included all but one of the activities defined as Very High ASP or High ASP , while all other programs usually provided only one or two of them. This result can offer interesting insights if taken together with the above-reported considerations on the importance to promote overall school improvement, and not to restrict the focus on one or few specific problems. On the basis of this result, we tentatively advance that when the study scope is broad and the theoretical approach is systemic, interventions are more directly centered on lending a voice to students and assigning to them a decision-making role. This again supports the importance of promoting whole-school interventions targeted toward the general learning environment.

Limitations and Conclusion

We are aware that the findings of the present literature review should be considered in the light of certain limitations. First of all, our choice to include only studies published in peer-reviewed journals in English requires some caution. While this selection criteria allowed us to provide a picture of the international literature, this might entail the loss of programs published in other languages that nonetheless contribute to the issue and deserve to be explored in future reviews. Secondly, in the present work, we did not address the issue of cross-cultural similarities and differences in schooling and education, which may influence the way ASP is conceived and valorized in the school context. However, the levels of ASP activities we proposed have the strength of resulting from the analysis of programs from several countries and may thus offer a basis for future discussions on the cross-cultural validity of practices fostering ASP. Furthermore, the present review has focused only on secondary school programs. While this choice was needed for guaranteeing clear references and boundaries to our findings, it also leaves to be explored whether our proposed classification of ASP activities could also be applied to younger students. Given the developmental and organizational differences between primary and secondary levels of education, this issue certainly merits further exploration in future reviews.

As the aim of our review was to provide an in-depth and reasoned description of existing studies based on ASP and of the activities adopted to promote the active role of students, testing the efficacy of these studies was beyond the scope of the present work. While future research may advance this line of inquiry, based on the evidence that different outcomes are considered in such a small number of studies, it seems premature to move toward extensive efficacy testing such as meta-analyses. Rather, at present, it is probably more feasible and desirable to have an increasing amount of literature focusing on ASP in whole-school interventions, to collect further evidence of robust programs and activities, especially with regard to high ASP. As a possible further research question in this direction, we suggest it would also be useful to assess whether the ways of actively involving students may change depending on the intervention target outcomes or on the number of students taking part in the activities. Lastly, while this is true for any review of the literature, it should nonetheless be acknowledged that our syntheses and reflections are dependent upon the choices made in the article selection process. For example, our inclusion and exclusion criteria (i.e., focusing on general participants and targets of intervention) may have restricted our sample. With this in mind, we followed closely the PRISMA guidelines and detailed each step of the process, so that readers can be well informed and future research may build as seamlessly as possible from our work.

Despite the mentioned limitations, this review provides a literature advance in its in-depth examination of existing whole-school interventions that include active student participation in secondary school. Their description and reasoned synthesis make available to researchers and practitioners an overview of specific programs and activities that are being used to actively involve students in processes of change. This in turn can inform reflections and experimentations as to how to integrate and improve the existing provision. In this direction, the major effort and contribution of the present review is the proposal of an organizing structure based on different levels of ASP for analyzing interventions, which allows to classify the specific activities included in each program. Such an effort provides a common ground for reflections and future studies on active student participation, as a shared classification can be instrumental for planning new interventions or evaluating the actual degree of students’ active involvement in the implemented programs. Overall, this work significantly contributes to the far-reaching goal of encouraging student participation in school life, and more specifically in the transformation of their learning environment, so that they can be empowered in shaping it to be increasingly responsive to their insights, ideas, and needs.

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All the authors contributed to the study conception and design. Literature search was conducted by Sara Berti; assessment of methodological quality was supervised by Valentina Grazia; draft of the introduction and overall supervision were performed by Luisa Molinari. All the authors contributed to data analyses, draft of the manuscript, and critical revisions of the work and they eventually approved the final manuscript.

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Berti, S., Grazia, V. & Molinari, L. Active Student Participation in Whole-School Interventions in Secondary School. A Systematic Literature Review. Educ Psychol Rev 35 , 52 (2023). https://doi.org/10.1007/s10648-023-09773-x

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DOI : https://doi.org/10.1007/s10648-023-09773-x

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