Each wave of the pandemic has underscored just how gravely contagious COVID-19 is, but there is less clarity among experts on exactly when and to what extent infected individuals are most likely to spread the virus.

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A new study by Boston University researchers has found that individuals infected with the virus are most contagious two days before, and three days after they develop symptoms. The study, published in the journal JAMA Internal Medicine, also found that infected individuals 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. "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," said Leonardo Martinez, Assistant Professor of epidemiology at the varsity's School of Public Health.

The team 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.

Of the individuals identified as primary cases, 89 per cent developed mild or moderate symptoms, and only 11 per cent 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. 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. "This study further emphasises the need for vaccination, which reduces clinical severity among people that develop COVID," Martinez said.