New research delivers an answer to a burning question: When are COVID-19 patients most infectious?
The answer? Two days before and three days after they develop symptoms.
The findings highlight the importance of rapid testing and quarantine if someone is feeling sick, the study authors say.
The researchers also found that infected people are more likely to be asymptomatic if they caught 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,” study co-leader Dr. Leonardo Martinez says.
“We wanted to see if results from these past studies, which show 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,” Martinez adds.
For the study, Martinez and his colleagues analyzed data from about 9,000 close contacts of primary cases in the Zhejiang province of China from January 2020 to August 2020. That’s before the advent of the even more contagious Delta variant.
Close contacts included people who lived in the same household or who dined together, co-workers, people in hospital settings and riders in shared vehicles. They were monitored for at least 90 days after their