The effectiveness of mask wearing at controlling severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission has been unclear. While masks are known to substantially reduce disease transmission in healthcare settings D. K. Chu et at, Lancet 395, 1973-1987 (2020); J. Howard et at, Proc. Nad. Acad. Set. U.S.A. 118, e20l4564l 18 (2021); Y. Cheng etal, Science eabg6296 (2021), studies in community settings report inconsistent results H. M. Ollila et ah, medRxiv (2020); J. Brainard et at, Eurosurveillance 25, 2000725 (2020); T. Jefferson et at, Cochrane Database Syst. Rev. 11, CD006207 (2020). Most such studies focus on how masks impact transmission, by analyzing how effective government mask mandates are. However, we find that widespread voluntary mask wearing, and other data limitations, make mandate effectiveness a poor proxy for mask-wearing effectiveness. We directly analyze the effect of mask wearing on SARS-CoV-2 transmission, drawing on several datasets covering 92 regions on six continents, including the largest survey of wearing behavior (n = 20 million) F. Kreuter et ah, https://gisumd.github.io/COVID-19-API-Documentation (2020). Using a Bayesian hierarchical model, we estimate the effect of mask wearing on transmission, by linking reported wearing levels to reported cases in each region, while adjusting for mobility and nonpharmaceutical interventions (NPIs), such as bans on large gatherings. Our estimates imply that the mean observed level of mask wearing corresponds to a 19 decrease in the reproduction number R. We also assess the robustness of our results in 60 tests spanning 20 sensitivity analyses. In light of these results, policymakers can effectively reduce transmission by intervening to increase mask wearing.
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