In a new study from Boston University, researchers found that people infected with the virus are most contagious two days before, and three days after, they develop symptoms.
They also found that infected people were more likely to be asymptomatic if they contracted the virus from a primary case (the first infected person in an outbreak) who was also asymptomatic.
In previous studies, viral load has been used as an indirect measure of transmission.
In the study, the team 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.
They conducted contact tracing and studied COVID-19 transmission among approximately 9,000 close contacts of primary cases in the Zhejiang province of China from January 2020 to August 2020.
“Close” contacts included household contacts (defined as individuals who lived in the same household or who dined together), co-workers, people in hospital settings, and riders in shared vehicles.
The researchers monitored infected individuals for at least 90 days after their initial positive COVID test results to distinguish between asymptomatic and pre-symptomatic cases.
Of the individuals identified as primary cases, 89% developed mild or moderate symptoms, and only 11% were asymptomatic—and no one developed severe symptoms.
Household members of primary cases, as well as people who were exposed to primary cases multiple times or for longer durations of time, had higher infection rates than other close contacts.
But regardless of these risk factors, close contacts were more likely to contract COVID-19 from the primary infected individual if they were exposed shortly before or after the individual developed noticeable symptoms.
These 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.
In comparison to mild and moderate symptomatic individuals, asymptomatic primary individuals were much less likely to transmit COVID to close contacts—but if they did, the contacts were also less likely to experience noticeable symptoms.
The team says the study further emphasizes the need for vaccination, which reduces clinical severity among people that develop COVID.
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The study is published in JAMA Internal Medicine. One author of the study is Dr. Leonardo Martinez.
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