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Approximately a third of COVID-19 infections are asymptomatic

Rod Tucker
26 January, 2021  

It has become widely accepted that a noticeable feature of COVID-19 infections is that a certain proportion are asymptomatic but the overall prevalence is unknown.

The presence of asymptomatic carriers of COVID-19 has been a cause for concern in the current pandemic due to the risk of these individuals, unknowingly transmitting the virus to others. Unfortunately, the only means of identifying asymptomatic carriers is through retrospective testing by which time, an individual could have passed the virus to many others.

The authors, from the Scripps Research Translational Institute, California, US, have previously examined the available literature in June 2020 on the prevalence of asymptomatic carriers. However, they report being aware that since then, a great deal of new information has become available, prompting an updated review. The authors have periodically searched the major databases as well as unpublished preprint servers such as MedRxiv for observational, descriptive and mass screening reports which involved COVID-19 testing via PCR or antibody methods. For the current review, they recorded the total number of people tested, positive results, the number of positive cases without symptoms and the nature of the data collection methods.

Findings
The authors identified 61 studies, 43 including PCR testing and 18 using antibody methods. The proportion of individuals testing positive with PCR tests but without symptoms at the time of the test, ranged from 6.3% to 100%, with a median of 65.9%. In 14 of the PCR studies, symptom data was collected longitudinally (ranging from 2 to 70 days) allowing for the distinction between pre-symptomatic and asymptomatic individuals. The proportion who tested positive but remained asymptomatic ranged from 11.1 to 100% with a median of 72.3%.

In discussing their findings, the authors identified the difficulty of distinguishing between between pre-symptomatic (i.e., those who go on to develop symptoms) and asymptomatic carriers, because testing is only done once. However, when using antibody tests, it is easier to distinguish between the two groups, particularly if data is gathered on symptoms at the time and before testing. The highest quality evidence on asymptomatic carriers was derived from two large surveys conducted in England and Spain. The English study identified a prevalence of 32.4% compared with 33% in the Spanish study. Based on these results, the authors estimated that prevalence of asymptomatic carriers is likely to be around a third.

Citation
Oran DP, Topal EJ. The proportion of SARS-CoV-2 infections that are asymptomatic. Ann Int Med 2021 doi.org/10.7326/M20-6976