Background: 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 last decade, there have been many advances in systematic review methodology and terminology, which have necessitated an update to the guideline.Objectives: To develop the PRISMA 2020 statement for reporting systematic reviews.Methods: We reviewed 60 documents with reporting guidance for systematic reviews to generate suggested modifications to the PRISMA 2009 statement. We sought feedback on the suggested modifications through an online survey of 110 systematic review methodologists and journal editors. The results of the review and survey were discussed at a 21-member in-person meeting. Following the meeting, drafts of the PRISMA 2020 checklist, abstract checklist, explanation and elaboration and flow diagram were generated and refined iteratively based on feedback from co-authors and a convenience sample of 15 systematic reviewers.Results: In this statement paper, 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. The checklist 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. The PRISMA 2020 statement replaces the 2009 statement.Conclusions: The PRISMA 2020 statement is intended to facilitate transparent, complete and accurate reporting of systematic reviews. Improved reporting should benefit users of reviews, including guideline developers, policy makers, health care providers, patients and other stakeholders. In order to achieve this, we encourage authors, editors and peer-reviewers to adopt the guideline.

To provide guidance on how systematic review authors, guideline developers, and health technology assessment practitioners should approach the use of the risk of bias in nonrandomized studies of interventions (ROBINS-I) tool as a part of GRADE's certainty rating process. The study design and setting comprised iterative discussions, testing in systematic reviews, and presentation at GRADE working group meetings with feedback from the GRADE working group. We describe where to start the initial assessment of a body of evidence with the use of ROBINS-I and where one would anticipate the final rating would end up. The GRADE accounted for issues that mitigate concerns about confounding and selection bias by introducing the upgrading domains: large effects, dose-effect relations, and when plausible residual confounders or other biases increase certainty. They will need to be considered in an assessment of a body of evidence when using ROBINS-I. The use of ROBINS-I in GRADE assessments may allow for a better comparison of evidence from randomized controlled trials (RCTs) and nonrandomized studies (NRSs) because they are placed on a common metric for risk of bias. Challenges remain, including appropriate presentation of evidence from RCTs and NRSs for decision-making and how to optimally integrate RCTs and NRSs in an evidence assessment.

Background There is a need for a comprehensive and critical review of the literature to inform scientific debates about the public health effects of waterpipe smoking. The objective of this study was therefore to systematically review the medical literature for the effects of waterpipe tobacco smoking on health outcomes. Methods We conducted a systematic review using the Cochrane Collaboration methodology for conducting systematic reviews. We rated the quality of evidence for each outcome using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) methodology. Results Twenty-four studies were eligible for this review. Based on the available evidence, waterpipe tobacco smoking was significantly associated with lung cancer [odds ratio (OR) = 2.12; 95% confidence interval (CI) 1.32–3.42], respiratory illness (OR = 2.3; 95% CI 1.1–5.1), low birth-weight (OR = 2.12; 95% CI 1.08–4.18) and periodontal disease (OR = 3–5). It was not significantly associated with bladder cancer (OR = 0.8; 95% CI 0.2–4.0), nasopharyngeal cancer (OR = 0.49; 95% CI 0.20–1.23), oesophageal cancer (OR = 1.85; 95% CI 0.95–3.58), oral dysplasia (OR = 8.33; 95% CI 0.78–9.47) or infertility (OR = 2.5; 95% CI 1.0–6.3) but the CIs did not exclude important associations. Smoking waterpipe in groups was not significantly associated with hepatitis C infection (OR = 0.98; 95% CI 0.80–1.21). The quality of evidence for the different outcomes varied from very low to low. Conclusion Waterpipe tobacco smoking is possibly associated with a number of deleterious health outcomes. There is a need for high-quality studies to identify and quantify with confidence all the health effects of this form of smoking.