By opting out of the state’s mask mandate, 81 counties in Kansas started an unintended experiment. COVID-19 incidence doubled in counties ignoring the mandate, but decreased about 6% in counties that required masks or other face coverings in public spaces.

Source: Trends in County-Level COVID-19 Incidence in Counties With and Without a Mask Mandate — Kansas, June 1–August 23, 2020 (MMWR Nov. 27, 2020)