The Centers for Disease Control and Prevention (CDC) recently published a paper reporting that Japanese data showed that coronavirus transmission was nearly 20 times more likely indoors than outdoors.
Environmental factors include population density and the availability and use of infection prevention and control measures in healthcare facilities. SARS and MERS had relatively low rates of person-to-person transmission but caused explosive outbreaks in healthcare settings (28). Rapid person-to-person transmission of COVID-19 appears likely to have occurred in healthcare settings, on a cruise ship, and in a church (3). In a study of 110 case-patients from 11 clusters in Japan, all clusters were associated with closed environments, including fitness centers, shared eating environments, and hospitals, the odds for transmission from a primary case-patient were 18.7 times higher than in open-air environments (H. Nishiura et al., unpub. data).
h/t Daniel Higdon