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OBJECTIVE: The Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement, first published in 2009, has been widely endorsed and compliance is high in systematic reviews of intervention studies. Systematic reviews of prevalence studies are increasing in frequency, but their characteristics and reporting quality have not been examined in large studies. Our objectives were to describe the characteristics of systematic reviews of prevalence studies in adults, evaluate the completeness of reporting and explore study-level characteristics associated with the completeness of reporting. STUDY DESIGN AND SETTING: We did a meta-research study. We searched 5 databases from January 2010 to December 2020 to identify systematic reviews of prevalence studies in adult populations. We used the PRISMA 2009 checklist to assess completeness of reporting and recorded additional characteristics. We conducted a descriptive analysis of review characteristics and linear regression to assess the relationship between compliance with PRISMA and publication characteristics. RESULTS: We included 1172 systematic reviews of prevalence studies. The number of reviews increased from 25 in 2010 to 273 in 2020. The median PRISMA score for systematic reviews without meta-analysis was 17.5 out of a maximum of 23 and, for systematic reviews with meta-analysis, 22 out of a maximum of 25. Completeness of reporting, particularly for key items in the methods section was suboptimal. Systematic reviews that included a meta-analysis or reported using a reporting or conduct guideline were the factors most strongly associated with increased compliance with PRISMA 2009. CONCLUSION: Reporting of systematic reviews of prevalence was adequate for many PRISMA items. Nonetheless, this study highlights aspects for which special attention is needed. Development of a specific tool to assess the risk of bias in prevalence studies and an extension to the PRISMA statement could improve the conduct and reporting of systematic reviews of prevalence studies.
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BACKGROUND: Debate about the level of asymptomatic Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection continues. The amount of evidence is increasing and study designs have changed over time. We updated a living systematic review to address 3 questions: (1) Among people who become infected with SARS-CoV-2, what proportion does not experience symptoms at all during their infection? (2) What is the infectiousness of asymptomatic and presymptomatic, compared with symptomatic, SARS-CoV-2 infection? (3) What proportion of SARS-CoV-2 transmission in a population is accounted for by people who are asymptomatic or presymptomatic? METHODS AND FINDINGS: The protocol was first published on 1 April 2020 and last updated on 18 June 2021. We searched PubMed, Embase, bioRxiv, and medRxiv, aggregated in a database of SARS-CoV-2 literature, most recently on 6 July 2021. Studies of people with PCR-diagnosed SARS-CoV-2, which documented symptom status at the beginning and end of follow-up, or mathematical modelling studies were included. Studies restricted to people already diagnosed, of single individuals or families, or without sufficient follow-up were excluded. One reviewer extracted data and a second verified the extraction, with disagreement resolved by discussion or a third reviewer. Risk of bias in empirical studies was assessed with a bespoke checklist and modelling studies with a published checklist. All data syntheses were done using random effects models. Review question (1): We included 130 studies. Heterogeneity was high so we did not estimate a mean proportion of asymptomatic infections overall (interquartile range (IQR) 14% to 50%, prediction interval 2% to 90%), or in 84 studies based on screening of defined populations (IQR 20% to 65%, prediction interval 4% to 94%). In 46 studies based on contact or outbreak investigations, the summary proportion asymptomatic was 19% (95% confidence interval (CI) 15% to 25%, prediction interval 2% to 70%). (2) The secondary attack rate in contacts of people with asymptomatic infection compared with symptomatic infection was 0.32 (95% CI 0.16 to 0.64, prediction interval 0.11 to 0.95, 8 studies). (3) In 13 modelling studies fit to data, the proportion of all SARS-CoV-2 transmission from presymptomatic individuals was higher than from asymptomatic individuals. Limitations of the evidence include high heterogeneity and high risks of selection and information bias in studies that were not designed to measure persistently asymptomatic infection, and limited information about variants of concern or in people who have been vaccinated. CONCLUSIONS: Based on studies published up to July 2021, most SARS-CoV-2 infections were not persistently asymptomatic, and asymptomatic infections were less infectious than symptomatic infections. Summary estimates from meta-analysis may be misleading when variability between studies is extreme and prediction intervals should be presented. Future studies should determine the asymptomatic proportion of SARS-CoV-2 infections caused by variants of concern and in people with immunity following vaccination or previous infection. Without prospective longitudinal studies with methods that minimise selection and measurement biases, further updates with the study types included in this living systematic review are unlikely to be able to provide a reliable summary estimate of the proportion of asymptomatic infections caused by SARS-CoV-2. REVIEW PROTOCOL: Open Science Framework (https://osf.io/9ewys/).