While some people who contract SARS-CoV-2 infections never experience any symptoms, there remains disagreement about what proportion of total infections these cases comprise. A study by researchers of the Institute of Social and Preventive Medicine of the University of Bern suggests that true asymptomatic cases of SARS-CoV-2 comprise a minority of infections.
The full spectrum and distribution of the severity of COVID-19 symptoms are not well understood. Some infected people may experience severe infections resulting in 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, researchers of the Institute of Social and Preventive Medicine (ISPM) of the University of Bern systematically reviewed literature using a database of SARS-CoV-2 evidence between January and June 2020. The researchers then analysed 79 studies reporting empirical data on about 6,616 people, 1,287 of whom were defined as asymptomatic in order to determine the proportion of infected people who never developed symptoms. The new study in PLOS Medicine estimated that about 20% of COVID-19 infections remained asymptomatic during follow up, based on all 79 studies. The study was limited in its inability to ascertain the impact of false negative test results, which could underestimate the proportion of asymptomatic infections.
Regular review updates in the future
Accurate estimations of true asymptomatic and presymptomatic infections are critical to understanding SARS-CoV-2 transmission at the population level and for populations to adopt appropriately tailored public health strategies. Future research should include prospective longitudinal studies that document symptom status. There is still quite high variability in the estimates from different studies, so the researchers will update their review regularly to incorporate new evidence as it emerges. Since each person infected with SARS-CoV-2 is initially asymptomatic, the proportion that will go on to develop symptoms is estimated to be around 80%. The people who are in the presymptomatic period are infectious for two or more days before they develop symptoms, so presymptomatic transmission may significantly contribute to overall SARS CoV-2 epidemics.
According to Nicola Low, senior author from the ISPM, “Systematic reviews that apply accurate definitions of truly asymptomatic infections are essential. Our review suggests that most SARS-CoV-2 infections are not asymptomatic throughout the course of infection.’
For this study, funding was received from the Swiss National Science Foundation, the European Union Horizon 2020 research and innovation programme, the Swiss government excellence scholarship, and the Swiss School of Public Health Global P3HS stipend.