Closed environments facilitate secondary transmission of coronavirus disease 2019 (COVID-19)

Commissioned by the Minister of the Ministry of Health, Labour, and Welfare of Japan, we collected secondary transmission data with the aim of identifying high risk transmission settings. We show that closed environments contribute to secondary transmission of COVID-19 and promote superspreading events. Closed environments are consistent with large-scale COVID-19 transmission events such as that of the ski chalet-associated cluster in France and the church- and hospital-associated clusters in South Korea. Our findings are also consistent with the declining incidence of COVID-19 cases in China, as gathering in closed environments was prohibited in the wake of the rapid spread of the disease. Reduction of unnecessary close contact in closed environments may help prevent large case clusters and superspreading events.