The Impact of State Mask-Wearing Requirements on the Growth of COVID-19 Cases in the United States.

In our ecologic analysis of US states, piecewise multivariable models showed lower post- vs. pre-mask case-rate slopes, with -1.08% per 100,000 per day (95% CI: -1.48%, -0.67%) among early- and -0.37% per 100,000 per day (95% CI: -0.86%, 0.10%) among late- versus never-adopter states. Our findings support statewide mask requirements to mitigate COVID-19 transmission.