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Disclosures: The authors report no relevant financial disclosures.
January 15, 2021
2 min read

Most SARS-CoV-2 transmission comes from asymptomatic people, CDC analysis finds

Disclosures: The authors report no relevant financial disclosures.
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A new analytical model suggests that more than half of all SARS-CoV-2 transmission comes from asymptomatic people, CDC researchers reported.

The finding, published in JAMA Network Open, indicates that isolating symptomatic patients with COVID-19 will not be enough to stop the spread of the virus, researchers said.

Jay Butler quote

“These findings underscore the importance of mitigation measures such as wearing masks, social distancing and washing of hands by everyone to slow spread of COVID-19, including from persons without symptoms,” CDC Deputy Director for Infectious Diseases Jay Butler, MD, told Healio. “They also suggest that strategic testing of persons without symptoms, such as those who have been exposed to COVID-19 or who have frequent contact with the public, can also reduce spread until safe and effective vaccines are widely available and used.”

Butler and colleagues used a decision analytical model to assess the relative SARS-CoV-2 transmission from presymptomatic, never symptomatic and symptomatic patients in a number of scenarios in which the proportion of transmission from asymptomatic individuals, as well as the infectious disease period, differed in published estimates.

“For all estimates, data from a meta-analysis was used to set the incubation period at a median of 5 days. The infectious period duration was maintained at 10 days, and peak infectiousness was varied between 3 and 7 days (2 and +2 days relative to the median incubation period),” the researchers explained. “The overall proportion of SARS-CoV-2 was varied between 0% and 70% to assess a wide range of possible proportions.”

The researchers found that 59% of all transmission came from asymptomatic individuals — 35% from presymptomatic individuals and 24% from persons who never developed symptoms.

Butler noted surprise that the finding that half of transmissions came from people without symptoms “held up under such a wide range of based values” and said transmission from people without symptoms “plays a significant role in the spread and continuation of the COVID-19 pandemic.”

“The findings are based on a mathematical model, and the quality of the model is only as good as the data that [go] into it. One of the strengths of the analysis is that we were able to vary the parameters based on findings from a number of studies, but as our knowledge of COVID-19 grows, we may need to update the analysis,” he said. “Additionally, the epidemiology of COVID-19 could change if SARS-CoV-2 variants with different clinical and epidemiological behaviors emerge.”