State policies mandating public or community use of face masks or covers in mitigating novel coronavirus disease (COVID-19) spread are hotly contested.

This study uses a natural experiment to examine whether statewide mandates to wear face masks in public affect the spread of COVID-19. The authors examined changes in the daily county-level COVID-19 growth rates between March 31, 2020 and May 22, 2020, a period in which 15 states issued mandates for face mask use in public. They evaluated data on statewide face covering mandate orders from public datasets on such policies and from searching and reviewing all state orders issued between April 1 and May 21, 2020.

Using a pre-post design that compared the pre-post mandate changes in COVID-19 spread in the states with mandates to the states that did not pass these mandates over time, they found evidence that mandating face mask use in public is associated with a decline in the daily COVID-19 growth rate. Specifically, they found that the average daily county-level growth rate decreased by 0.9, 1.1, 1.4, 1.7, and 2.0 percentage-points in 1–5, 6–10, 11–15, 16–20, and 21+ days after signing, respectively, corresponding to 230,000–450,000 averted COVID-19 cases by May 22, 2020.

The findings suggest that requiring face mask use in public might help in mitigating COVID-19 spread.