A far-reaching study of SARS-CoV-2 transmission in China’s Hunan Province found that the encounters most likely to spread the coronavirus were those between members of the same household.

Kaiyuan Sun at the National Institutes of Health in Bethesda, Maryland, Hongjie Yu at Fudan University in Shanghai, China, and their colleagues analyzed data from 1,178 people in Hunan who were infected with SARS-CoV-2 and more than 15,000 close contacts of the infected people. The team found that communications between people who live together posed the most significant risk of transmission, followed by contacts between extended family members. The transmission risk was lower for social connections and community encounters, such as those on public transport. Every extra day of contact raised transmission risk by 10%, the team found.