Estimating the Extent of True Asymptomatic COVID-19 and Its Potential for Community Transmission: Systematic Review and Meta-Analysis

Background: The prevalence of true asymptomatic COVID-19 cases is critical to policy makers considering the effectiveness of mitigation measures against the SARS-CoV-2 pandemic. We aimed to synthesise all available research on the asymptomatic rates. Methods: We searched PubMed, Embase, Cochrane COVID 19 trials, and European PMC for pre-print platforms such as MedRxiv, Research Square, and F1000 Research. We included primary studies reporting on asymptomatic prevalence where: (a) the sample frame was not contingent on the presence or absence of symptoms, and (b) there was sufficiently long follow up to identify pre-symptomatic cases. Meta-analysis used fixed effect and random effects models. Results: We screened 571 articles and included five studies from three countries (China (2), USA (2), Italy (1)) that include 599 COVID-19 cases and 9,297 contacts. Diagnosis in all studies was confirmed using a RT-PCR test. The proportion of asymptomatic cases ranged from 6% to 41%. Meta-analysis (fixed effect) found that the proportion of asymptomatic cases was 16% (95% CI: 12% - 20%) overall; higher in non-aged care 19% (15% - 24%), and lower in long-term aged care 8% (4% - 14%). Two studies provided direct evidence of forward transmission of the infection by asymptomatic cases, but suggested lower rates than symptomatic cases. Conclusion: Our meta-analytic estimates of the prevalence of asymptomatic COVID-19 cases are lower than many highly publicized studies, but still substantial. Further robust epidemiological evidence is urgently needed, including in sub-populations such as children, to better understand the importance of asymptomatic cases for driving spread of the pandemic. Funding Statement: OB is supported by NHMRC Grant APP1106452. PG is supported by NHMRC Australian Fellowship grant 1080042. KB was supported by NHMRC Fellowship grant 1174523 and Program grant 1113532. There was no funding source for this study. Declaration of Interests: Prof Mary-Louise McLaws is a member of World Health Organization Health Emergencies Program Experts Advisory Panel for Infection Prevention and Control Preparedness, Readiness and Response to COVID-19. All other authors declare no competing interests.