COVID-19 is most transmissible 2 days before, 3 days after symptoms appear

BOSTON, Mass. — COVID-19 is very contagious. That much isn’t up for debate. However, the exact period when infected individuals are at their most contagious has remained unclear. Now, a new study is shedding some much needed light on the matter. Boston University researchers report COVID-19 positive individuals are at their most contagious two days before and three days after symptoms appear.

Additionally, the study also finds infected individuals are more likely to be asymptomatic if they contracted the coronavirus from a primary case (the first infected person in an outbreak) who also happens to be asymptomatic.

“In previous studies, viral load has been used as an indirect measure of transmission,” says study co-leader Dr. Leonardo Martinez, assistant professor of epidemiology at BUSPH, in a university release. “We wanted to see if results from these past studies, which show that that COVID cases are most transmissible a few days before and after symptom onset, could be confirmed by looking at secondary cases among close contacts.”

The team performed contact tracing and studied COVID-19 transmission rates among roughly 9,000 close contacts of primary cases living in the Zhejiang province of China between January and August 2020. Researchers defined a “close contact” as anyone who lives or eats together, co-workers, people exposed to hospital settings, and riders in shared vehicles. Scientists also tracked infected individuals for a minimum of 90 days after testing positive for COVID-19 in order to differentiate between asymptomatic and pre-symptomatic cases.

Timing (of exposure) is everything for COVID transmission

Among those identified as primary cases, 89 percent went on to develop mild or moderate symptoms, while 11 percent were asymptomatic. No one developed severe symptoms. Household members of primary cases, in addition to those exposed to primary cases on more than one occasion or for a long time, showed higher infection rates than all other close contacts. However, above all of those risk factors, close contacts were more likely to contract COVID-19 from a primary infected individual if they encountered that person shortly before or after the individual showed noticeable symptoms.

“Our results suggest that the timing of exposure relative to primary-case symptoms is important for transmission, and this understanding provides further evidence that rapid testing and quarantine after someone is feeling sick is a critical step to control the epidemic,” Dr. Martinez adds.

Asymptomatic individuals were much less likely to infect others than symptomatic individuals. And, even if a symptomatic carrier did infect someone else, that person was less likely to experience any obvious symptoms themselves.

“This study further emphasizes the need for vaccination, which reduces clinical severity among people that develop COVID,” Dr. Martinez concludes.

The findings appear in the journal JAMA Internal Medicine.

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John Anderer

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