evidence: How Effective are Mask Mandates at Stopping the Spread of COVID-19? [...] Moreover, a mask mandate may have different effects depending on the characteristics of the community in which it is enacted (e.g., population density, acceptance of mask wearing). Therefore, for every county we included in our study, we used a matching strategy to find a control county that had similar population density, acceptance towards mask wearing, and number of cases, but no mask mandate. We ended up with two groups of counties that were comparable in demographics and pandemic severity, but one group had mask mandates during our study period while the other did not. We then followed the two groups for a few weeks to evaluate the effect of the mandate by comparing the difference between paired counties. [...] What we found from this study was that counties that introduced a mask mandate early on in the pandemic experienced lower county-level COVID-19 case incidence in the six weeks following enactment as compared to similar counties without a mandate. How much the mask mandate benefited communities varied across counties and over time, with the strongest effects seen in more crowded communities or those with a reticence toward voluntary masking. We also observed that case incidence declined the most between the third and fourth weeks after the mandate started and the effect of the mandate waned between weeks six and eight in some counties.