Outdoor Transmission of SARS-CoV-2 and Other Respiratory Viruses: A Systematic Review

Background: While risk of outdoor transmission of respiratory viral infections is hypothesized to be low, there are limited data on SARS-CoV-2 transmission in outdoor compared to indoor settings.

Methods: We conducted a systematic review of peer-reviewed papers indexed in PubMed, EMBASE, and Web of Science and preprints in Europe PMC through 12 August 2020 that described cases of human transmission of SARS-CoV-2. Reports of other respiratory virus transmission were included for reference.

Results: Five identified studies found a low proportion of reported global SARS-CoV-2 infections occurred outdoors (<10%) and the odds of indoor transmission was very high compared to outdoors (18.7 times; 95% confidence interval, 6.0-57.9). Five studies described influenza transmission outdoors and 2 adenovirus transmission outdoors. There was high heterogeneity in study quality and individual definitions of outdoor settings, which limited our ability to draw conclusions about outdoor transmission risks. In general, factors such as duration and frequency of personal contact, lack of personal protective equipment, and occasional indoor gathering during a largely outdoor experience were associated with outdoor reports of infection.

Conclusions: Existing evidence supports the wide-held belief that risk of SARS-CoV-2 transmission is lower outdoors but there are significant gaps in our understanding of specific pathways.