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While some people who get SARS-CoV-2 infections never experience any symptoms, there is disagreement about the proportion of total infections these cases represent. A new study published in the open access journal PLOS Medicine by Diana Buitrago-García of the University of Bern, Switzerland and her colleagues, suggests that true asymptomatic cases of SARS-CoV-2 comprise a minority of infections.
The full spectrum and severity distribution of COVID-19 symptoms is not well understood. Some infected people may experience serious infections leading to viral pneumonia, respiratory distress syndrome, and death, while others remain completely asymptomatic or develop mild, nonspecific symptoms. To better understand the proportion of people who become infected with SARS-CoV-2 and never develop any symptoms, as well as the proportion of people who are asymptomatic at the time of diagnosis but develop symptoms later, the researchers systematically reviewed the literature using a SARS-CoV-2 Evidence database between March and June 2020. The authors then analyzed 79 studies that reported empirical data on 6,616 people, 1,287 of whom were defined as asymptomatic, to determine the proportion of people infected they never developed symptoms. While the study was limited by its inability to determine the impact of false negatives, the researchers were able to estimate that 20% (95% CI 17-25) of COVID-19 infections remained asymptomatic during follow-up.
Accurate estimates of true asymptomatic and presymptomatic infections are critical to understanding the transmission of SARS-CoV-2 at the population level and for populations to adopt appropriately tailored public health strategies. Future research should include prospective longitudinal studies documenting symptom status. Greater precision of serological tests is also needed to reduce the number of false negatives. Since each person infected with SARS-CoV-2 is initially asymptomatic, the proportion who will develop symptoms is estimated to be around 80%, suggesting that presymptomatic transmission may contribute significantly to general SARS CoV-2 epidemics.
According to the authors, “The findings of this systematic review of publications at the onset of the pandemic suggest that the majority of SARS-CoV-2 infections are not asymptomatic during the course of infection. The contribution of presymptomatic and asymptomatic infections to the general transmission of SARS-CoV-2 means that combined prevention measures will continue to be needed, with improved respiratory and hand hygiene, testing and tracking, and isolation and social distancing strategies.
