Table 1.

Epidemiologic and Observational Studies Showing the Effectiveness of Masks in Reducing COVID-19 Transmission

Setting	Exposure of interest	Effect
USS Theodore Roosevelt aircraft carrier	face coverings during an outbreak	service members who wore face coverings had lower infection rate than those who did not (55.8% versus 80.8%)
Hair salons in Missouri	two masked hair stylists infected with COVID-19 exposed 139 clients, all masked	none of the 139 clients developed symptoms with 67 testing negative for SARS-CoV-2
Boston health care settings	institution of universal surgical masking with provision in hospitals	significantly lower rate of SARS-CoV-2 positivity among health care workers after masking
Arizona during summer surge	mask mandates, limiting large crowds, social distancing	transmission rates were up by 151% prior to these measures and then stabilized and decreased by 75% with continued application
Kansas counties during summer surge	state mask mandate with option for counties to opt-out in Kansas	COVID-19 incidence decreased in 24 counties with mask mandates after July 3, but continued to increase in 81 counties without mask mandates4
Tennessee counties	mask requirements	areas with mask requirements had a slower growth rate in hospitalizations for COVID-19 (without controlling for cases) than those without mask requirements6
States in the U.S.	mask mandates in 15 states and Washington, DC over summer	reduction in COVID-19 transmission rates in states mandating face mask use in public compared to those without mandates
Germany	regional mandates for mandatory mask wearing in public transport and shops	face masks reduced the number of new COVID-19 infections 45% (between 15% and 75%) over a period of 20 days after the mandates5

∗Unless indicated, all references are found in the CDC Scientific Brief²