Individuals with COVID-19 are most likely to spread the virus to close contacts two days before the onset of symptoms to three days after symptoms appear, and the risk of transmission is highest when patients had mild or moderate disease severity, according to a new study by researchers at the University of Georgia.

The study, which was published this week in JAMA Internal Medicine, supports the idea that if a person with COVID-19 is sicker, they are more contagious compared to asymptomatic cases.

The findings provide further evidence for interventions like contact tracing, masking and vaccines, says lead author Yang Ge, a doctoral student in UGA's College of Public Health.

"We found asymptomatic cases had lower transmissibility compared to symptomatic cases and were less likely to infect their contacts. In addition, we found that contacts that developed COVID-19 infections were more likely to be asymptomatic if they were exposed to an asymptomatic case," said Ge.

"This suggests interventions like vaccines and masking should continue to be encouraged."

Vaccines not only protect the vaccinated individual, but they also work to suppress the amount of virus that close contacts could be exposed to, and masking reduces the spread of aerosolized particles that could contain the virus.

The research team drew its findings from a large cohort study of 730 individuals who received a COVID-19 diagnosis in Zhejiang Province, China, between Jan. 8, 2020, and July 30, 2020.

Using detailed health records and contact tracing, the team was able to apply state-of-the-art analytical approaches to determine how the timing of exposure and disease severity impacted the risk of transmission.

The cohort included 8,852 close contacts, defined as members of a household, coworkers, and those exposed in a health care setting or shared transit.

To date, this is one of the largest contact tracing studies of its kind, said corresponding author Ye Shen, an associate professor of epidemiology and biostatistics in public health.

Though Shen says that these results need to be repeated in vaccinated populations, the study identifies a high-risk transmission window to help local municipal and public health officials target contact tracing efforts.

The study was co-authored by researchers at UGA, Boston University School of Public Health, Zhejiang Provincial Center for Disease Control and Prevention, University of Texas School of Public Health, and Tulane University School of Public Health and Tropical Medicine.