Mask mandates save lives.

We quantify the effect of statewide mask mandates in the United States in 2020. Our regression discontinuity design exploits county-level variation in COVID-19 outcomes across the border between states with and without mandates. State mask mandates reduced new weekly COVID-19 cases, hospital admissions, and deaths by 55, 11, and 0.7 per 100,000 inhabitants on average. The effect depends on political leaning with larger effects in Democratic-leaning counties. Our results imply that statewide mandates saved 87,000 lives through December 19, 2020, while a nationwide mandate could have saved 57,000 additional lives. This suggests that mask mandates can help counter pandemics, particularly if widely accepted.