COVID-19 most-transmissible two days before, three days after symptoms appear
People infected with the SARS-CoV-2 virus, which causes COVID-19, are most contagious two days before, and three days after they develop symptoms, according to a study conducted in China. The research, 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.
Previously, viral load was used as indirect measure of transmission
"In previous studies, viral load has been used as indirect measure of transmission," said Leonardo Martinez, assistant professor at Boston University School of Public Health, US. "We wanted to see if results from past studies, which show that COVID-19 cases are most transmissible a few days before and after symptom onset, could be confirmed by looking at secondary cases among close contacts," Martinez said.
Researchers studied COVID-19 transmission among 9,000 close contacts
The researchers 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.
Researchers monitored infected individuals for at least 90 days
The researchers, including the study co-lead Yang Ge from the University of Georgia College of Public Health, US, monitored infected individuals for at least 90 days after their initial positive COVID-19 test results to distinguish between asymptomatic and pre-symptomatic cases.
Household members of primary cases had higher infection rates
Of the individuals identified as primary cases, 89 percent developed mild or moderate symptoms, and only 11 percent 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.
Rapid testing and quarantine is necessary to control COVID-19: Martinez
However, 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. "Our results suggest that the timing of exposure relative to primary-case symptoms is important for transmission. Therefore, rapid testing and quarantine is a critical step to control the epidemic," Martinez said.
This study further emphasizes the need for vaccination: Martinez
In comparison to mild and moderate symptomatic individuals, asymptomatic primary individuals were much less likely to transmit COVID-19 to close contacts. "This study further emphasizes the need for vaccination, which reduces clinical severity among people that develop COVID-19," Martinez added.