Effects of Face Mask Mandates on COVID-19 Transmission in 51 Countries: Retrospective Event Study

Background: The utility of face masks in preventing acute respiratory infections has been debated, with inconclusive evidence from randomized controlled trials and a lack of real-world evidence with high external validity. Objective: To assess whether face mask mandates in 51 countries and recommendations in 10 countries increased self-reported mask use and reduced SARS-CoV-2 reproduction numbers (Rt) and COVID-19 case growth rates. Methods: A retrospective event study pooled four data sources: (1) self-reported mask use from the COVID-19 Trends and Impact Survey, (2) mask policies and other nonpharmacological interventions from the Oxford COVID-19 Government Response Tracker, (3) mobility indicators from Google Community Mobility Reports, and (4) Rt and case growth rates from Our World in Data. Models included country and time fixed effects, controls for other interventions, mobility, and testing, and Sun–Abraham interaction-weighted estimators of dynamic treatment effects. Results: Mandates increased mask use by 8.81 percentage points (P=.006) and reduced Rt by 0.31 units (P=.008). No significant average effect on case growth rates was found (−0.98 percentage points; P=.56), though incremental reductions appeared on days 26, 27, 29, and 30 after implementation. Incremental effects on mask use and Rt emerged by days 6 and 13, respectively, and persisted beyond 30 days. Recommendations increased mask use by 5.84 percentage points (P<.001) but showed no average effects on Rt (−0.06; P=.70) or case growth (−2.45 percentage points; P=.59), despite isolated incremental effects on select days.