In the 14 studies with longitudinal data that reported information on the evolution of symptomatic status, nearly three quarters of persons who tested positive but had no symptoms at the time of testing remained
Current data suggest that infected persons without symptoms—including both presymptomatic and asymptomatic persons—account for more than 40% of all SARS-CoV-2 transmission.
The proportion of new infections caused by asymptomatic persons alone
is uncertain, but when researchers in Wanzhou, China, analyzed epidemiologic data for “183 confirmed COVID19 cases and their close contacts from five generations of transmission,” they determined that the asymptomatic cases, which made up 32.8% of infected persons, caused
19.3% of infections.
The 61 studies and collected reports provide compelling evidence that the asymptomatic fraction of SARS-CoV-2 infection is sizable. These data
enable us to make reasonable inferences about the proportion of SARS-CoV-2 infections that are asymptomatic.
Studies designed to achieve representative samples of large populations provide useful data because they may accurately reflect human populations in general.
Four of the PCR-based studies are in this category, with target populations of England, Iceland, and Indiana. The proportion of persons who tested
positive but had no symptoms at the time of testing ranged from 43.0% to 76.5%, with a median of 45.6% (IQR, 43.6% to 61.8%). However, these studies fall shortof providing the highest-quality evidence because they
collected only cross-sectional data. As a result, it’s not possible to distinguish between presymptomatic and asymptomatic cases.
In 14 longitudinal studies that reported information on the evolution of symptomatic status, a median of 72.3% of persons who tested positive but had no symptoms at the time of testing remained asymptomatic during a follow-up period. If a similar proportion remained asymptomatic in the 4 large, representative, PCR-based studies, in which the median was 45.6%, the asymptomatic fraction of SARS-CoV-2 infection would be 33.0%.
Among the data assembled in this paper , the highest-quality evidence comes from the large-scale studies using antibody testing that were designed to achieve representative samples of nationwide populations in
England (n= 365 104) and Spain (n= 61 075). It is remarkable that these independently conducted serosurveys yielded nearly identical proportions of asymptomatic SARS-CoV-2 infections: 32.4% in England and 33.0% in
It can be inferred that persons who receive positive antibody test results can be classified accurately as asymptomatic because such results are likely to occur only after the onset of symptoms, if any.
In a study of 222 hospitalized patients in Wuhan, China, IgM and IgG antibodies to SARS-CoV-2 were first detected 3 and 4 days, respectively, after symptomatic onset of COVID-19. In a study of 109 health care workers and 64 hospitalized patients in Zurich, Switzerland, the severity of illness
seemed to affect how quickly SARS-CoV-2 antibodies appeared. Patients with severe COVID-19 had detectable SARS-CoV-2 antibody titers after symptom onset, but those with mild cases “remained negative or
became positive [for SARS-CoV-2 antibodies] 12 to 14 days after symptom onset”.
These data suggest that positive antibody test results are unlikely to occur during the period when it is uncertain whether an infected person is presymptomatic or asymptomatic.