Proportion of SARS-CoV-2 Infections That Are Asymptomatic

Quick Takes

  • The proportion of asymptomatic persons with SARS-CoV-2 is wide-ranging, but substantial, representing close to 50% of infected person’s studies using random sampling.
  • Most studies were cross-sectional in nature, limiting deriving an accurate estimate of persons that subsequently remained asymptomatic.
  • Policies surrounding COVID-19 control should account for the prevalence and transmission risk of asymptomatic SARS-CoV-2 infection.

Study Questions:

What is the proportion of people with a positive severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) test who are asymptomatic?


The authors performed a systematic review of the published literature through November 17, 2020, and identified 61 studies on mass screening for SARS-CoV-2, of which 14 were longitudinal. Forty-three studies used nucleic acid testing as the diagnostic method, and the remainder used serology. Thirty studies included a list of specific symptoms. The evidence was summarized as number of studies and the range, median, and interquartile range (IQR) for persons who tested positive but had no symptoms at the time of polymerase chain reaction (PCR) testing or who reported having had no symptoms before or at the time of antibody testing.


Among the 43 studies using PCR testing, the proportion of persons who tested positive but had no symptoms at the time of testing ranged from 6.3% to 100%, with a median of 65.9% (interquartile range [IQR], 42.8-87.0%). In the 14 longitudinal studies, the proportion of persons who tested positive and remained asymptomatic ranged from 6.3%–91.7%, with a median of 42.5% (IQR, 29.6%–77.8%). When limiting to the four studies, which used random selection of participants to achieve a representative sample, the proportion of asymptomatic persons ranged from 43.0%–76.5%. The largest study, which included 932,072 persons aged ≥5 years who underwent systematic testing in England, identified 47.0% amongst those positive for SARS-CoV-2 who had no symptoms. Another study from England, including 36,061 persons tested for SARS-CoV-2, reported a much larger proportion of asymptomatic individuals testing positive (74.8%). Similar wide ranges of the proportion of asymptomatic patients were reported in studies using antibody testing, without clear differences related to age to density of the environment test.


A substantial but wide-ranging proportion of persons with a SARS-CoV-2 infection are asymptomatic.


While a significant number of studies included in this systematic review have limitations, the estimates derived from representative, randomly selected population samples suggest at least one third of persons infected with SARS-CoV-2 are asymptomatic. This important, and now well-established fact questions the usefulness of screening strategies based on symptoms. However, justifying systematic and mass screening for SARS-CoV-2 based on the high proportion of asymptomatic infections is difficult, given the high false-negative rate of testing in asymptomatic patients. One essential component in devising an appropriate strategy to control the spread of COVID-19, and that remains poorly understood, is the rate of asymptomatic transmission—which appears to be low, with however widely varying reports. What is however unequivocal is the effectiveness of the COVID-19 vaccine, which should be the main focus of any policy surrounding COVID-19.

Clinical Topics: Prevention

Keywords: ACC COVID-19 Podcast, Antibodies, Asymptomatic Infections, Coronavirus, COVID-19, Mass Screening, Polymerase Chain Reaction, Primary Prevention, SARS-CoV-2, Vaccines

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