Estimating the extent of asymptomatic COVID-19 and its potential for community transmission: Systematic review and meta-analysis

Abstract

Background: Knowing the prevalence of true asymptomatic coronavirus disease 2019 (COVID-19) cases is critical for designing mitigation measures against the pandemic. We aimed to synthesize all available research on asymptomatic cases and transmission rates. Methods: We searched PubMed, Embase, Cochrane COVID-19 trials, and Europe PMC for primary studies on asymptomatic prevalence in which (1) the sample frame includes at-risk populations and (2) follow-up was sufficient to identify pre-symptomatic cases. Meta-analysis used fixed-effects and random-effects models. We assessed risk of bias by combination of questions adapted from risk of bias tools for prevalence and diagnostic accuracy studies. Results: We screened 2,454 articles and included 13 low risk-of-bias studies from seven countries that tested 21,708 at-risk people, of which 663 were positive and 111 asymptomatic. Diagnosis in all studies was confirmed using a real-time reverse transcriptase–polymerase chain reaction test. The asymptomatic proportion ranged from 4% to 41%. Meta-analysis (fixed effects) found that the proportion of asymptomatic cases was 17% (95% CI 14% to 20%) overall and higher in aged care (20%; 95% CI 14% to 27%) than in non-aged care (16%; 95% CI 13% to 20%). The relative risk (RR) of asymptomatic transmission was 42% lower than that for symptomatic transmission (combined RR 0.58; 95% CI 0.34 to 0.99, p = 0.047). Conclusions: Our one-in-six estimate of the prevalence of asymptomatic COVID-19 cases and asymptomatic transmission rates is lower than those of many highly publicized studies but still sufficient to warrant policy attention. Further robust epidemiological evidence is urgently needed, including in subpopulations such as children, to better understand how asymptomatic cases contribute to the pandemic.

Community prevalence of SARS-CoV-2 in England from April to November, 2020: results from the ONS Coronavirus Infection Survey

“… 28 A meta-analysis of studies focusing on close contacts of confirmed COVID-19 cases suggested that only 17% (95% CI 14–20) of infected individuals are asymptomatic. 29 However, informing people that they were recently in close contact with a confirmed COVID-19 case can result in recall bias and overestimate the true prevalence of symptoms among a representative sample of infected people. Although we might have underestimated the true prevalence of symptoms among people with SARS-CoV-2 infection in the community, partly due to asking about current symptoms at visits up to July 23, 2020 (meaning that very transient symptoms only occurring between visits would have been missed), and symptoms in the past 7 days thereafter, our study adds to the growing evidence that a substantial proportion of SARS-CoV-2 in the community could be asymptomatic.…”

SARS-Cov-2 viral and serological screening of staff in 31 European fertility units

“…The proportion of asymptomatic infections in different studies has varied greatly ranging from 4% to 41%, potentially reflecting differences in the follow-up of asymptomatic cases and populations studied ( Buitrago-Garcia et al , 2020 ; Byambasuren et al , 2020 ). Our observed frequency of ∼33% asymptomatic cases is similar; however, we acknowledge the limitations of symptom recall may have had on accurate ascertainment.…”

“…Our observed frequency of ∼33% asymptomatic cases is similar; however, we acknowledge the limitations of symptom recall may have had on accurate ascertainment. Onward secondary infection transmission from asymptomatic cases ranges from none to 2.2%, as compared to symptomatic cases where transmission rates range between 0.8% and 15.4% ( Byambasuren et al , 2020 ). That the virus was detectable before the onset of symptoms further highlights the disadvantages of relying solely on questionnaire triage.…”

McQ – An open-source multiplexed SARS-CoV-2 quantification platform

“…One of the challenges in the control of the COVID-19 pandemic is the high frequency of pre-or asymptomatic individuals that nevertheless display high levels of infectivity, making containment of transmission solely by symptomatic testing impossible [4][5][6] . This is the case for a majority of patients during the 2-7 days of the incubation phase 7 and lasts during the entire duration of the infection for approximately one fifth of individuals 8 . Detection of these asymptomatic spreaders could be achieved by the implementation of highfrequency population-scale surveillance testing.…”