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SARS-CoV-2 Transmission From People Without COVID-19 Symptoms
Quick Takes
- Spread of SARS-CoV-2 from pre-symptomatic and asymptomatic individuals is estimated to account for half of all transmissions.
- Using the most conservative assumptions of peak infectiousness of 2 days and 0% transmission from never symptomatic individuals, asymptomatic transmission is estimated to be at least 25%.
- The findings of this study emphasize the importance of wearing masks and social distancing regardless of symptoms to curb the spread of SARS-CoV-2.
Study Questions:
What is the estimated proportion of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmissions that occur from asymptomatic individuals?
Methods:
In this study by researchers from the Centers for Disease Control and Prevention (CDC), the authors use a simple model based on data from a meta-analysis of eight studies from China to assess the proportion of SARS-CoV-2 transmission from pre-symptomatic, asymptomatic, and symptomatic individuals. Assumptions based on the data include a median incubation period of 5 days, with an infectious period of 10 days and 95% of symptomatic individuals developing symptoms by day 12. Peak infectiousness was assumed to occur at day 5. These baseline assumptions imply that a person with infection who never develops symptoms can account for approximately 24% of transmissions. The authors tested the model across a wide range of possible proportions of asymptomatic individuals and the relative importance of different levels of asymptomatic and pre-symptomatic transmission.
Results:
Under baseline assumptions, approximately 59% of all transmissions are estimated to have originated from asymptomatic transmission (pre-symptomatic + never symptomatic): 35% from pre-symptomatic individuals and 24% from individuals who are never symptomatic. Shifting the different elements of the model (peak transmission date, proportion of transmission from never symptomatic individuals, median symptom onset) yielded total asymptomatic transmission estimates ranging from 50% to 67%. Using the very conservative assumption of peak infectiousness of 2 days and 0% transmission from never symptomatic individuals still resulted in more than 25% transmission from asymptomatic individuals.
Conclusions:
SARS-CoV-2 transmission from asymptomatic individuals is estimated to account for at least 50% of all transmissions.
Perspective:
Controlling the spread of SARS-CoV-2 has proven to be a monumental challenge for many reasons; one of them being with the high proportion of asymptomatic transmission. While this study by the CDC uses a relatively simple model to estimate a complex phenomenon, the results are consistent with prior independent analyses using contact tracing data, which found also estimated asymptomatic transmission to be close to 50%. Other studies have shown that symptomatic individuals are more likely to transmit the virus; however, screening for and isolating symptomatic cases is unlikely to be effective as a sole strategy for control of the spread of SARS-CoV-2, and should be complemented by measures such as mask wearing and social distancing, regardless of symptoms.
Clinical Topics: COVID-19 Hub, Prevention
Keywords: Asymptomatic Diseases, Contact Tracing, COVID-19, Disease Transmission, Infectious, Primary Prevention, severe acute respiratory syndrome coronavirus 2
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