“In previous studies, viral load has been used as an indirect measure of transmission,” said researcher Dr. Leonardo Martinez. “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.”
Understanding the spread of the virus
For the study, the researchers performed contact tracing for nearly 9,000 people who were considered close contacts of those who had been infected with COVID-19. They monitored their symptoms over the course of three months and studied how the virus spread among the participants’ own network of close contacts.
The researchers learned that timing played a large role in how the virus spread. The study showed that people living in the same house as the infected person were the most likely to contract the virus, but other close contacts were likely to become infected depending on when they were exposed. The days right before and right after symptoms appeared were when the virus was the most contagious.
In terms of asymptomatic cases, the researchers learned that these participants were less likely to spread the virus to others; however, when they did, those cases were also more likely to be 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 Dr. Martinez.