Researchers from Southern Medical University, Guangzhou, China traced more than 3,410 close contacts of 391 COVID-19 index cases between January and March 2020 to evaluate the risk for disease transmission in different settings. They found that risk for secondary transmission of COVID-19 was less than 4 percent among close contacts of persons with COVID-19. In addition, secondary infections acquired while using public transportation were rare. In contrast, 1 in 10 household contacts was found to be infected.

The researchers also found that patients with more clinically severe disease were more likely to infect their close contacts than were less severe index cases. Those with asymptomatic cases were the least likely to infect their close contacts. Manifestation of certain symptoms, such as expectoration, in index cases was also associated with an increased risk for infection in their close contacts.

Reference: Luo L, et al. Contact Settings and Risk for Transmission in 3410 Close Contacts of Patients With COVID-19 in Guangzhou, China: A Prospective Cohort Study. Ann Intern Med. https://www.acpjournals.org/doi/10.7326/M20-2671