When is an individual most contagious with COVID-19?

FRIDAY, Aug 27, 2021 (HealthDay News) – New research provides an answer to a burning question: When are COVID-19 patients most contagious?

The answer? Two days before and three days after they develop symptoms.

The results underscore the importance of rapid testing and quarantine when someone feels sick, the study authors said.

The researchers also found that infected people were more likely to be asymptomatic if they caught the virus from a primary case (the first person infected in an outbreak) who was also asymptomatic.

“In previous studies, viral load was used as an indirect measure of transmission,” said study co-leader Dr. Leonardo Martinez, Assistant Professor of Epidemiology in the School of Public Health at Boston University.

“We wanted to see if the results of these previous studies, which show that COVID cases are most transmissible a few days before and after the onset of symptoms, could be confirmed by examining secondary cases from close contacts,” Martinez said in one University press release.

For the study, Martinez and his colleagues analyzed data from around 9,000 close contacts of primary cases in the Chinese province of Zhejiang from January 2020 to August 2020. That was before the even more contagious Delta variant appeared.

Close contacts included people who lived in the same household or dined together, work colleagues, people in hospitals and passengers in shared vehicles. They were monitored for at least 90 days after their first positive COVID-19 test results.

Of the primary cases, 89% developed mild or moderate symptoms, 11% were asymptomatic, and none had severe symptoms.

Household contacts of primary cases and people who were exposed to primary cases multiple times or for a longer period of time had higher infection rates than other close contact persons, the results showed.

However, according to the study published online on Aug. 23 in JAMA Internal Medicine, all close contacts were more likely to be infected from the primary case if exposed shortly before or after that person developed symptoms.

Compared to mild and moderate symptomatic primary cases, those who were asymptomatic infected close contacts much less often. When this was the case, the contacts also had less noticeable symptoms, the researchers reported.