Study population and setting
Using seven-day moving averages of COVID-19 incidence (number of cases per 100,000 persons) across 105 Kansas counties between June 1 and August 23, 2020, trends in COVID-19 incidence were compared in counties adopting a state-implemented mask mandate on July 3, 2020, to counties that opted out of the state mandate. A difference-in-differences analysis was used to estimate the impact of the mask mandates, comparing the rates of new COVID-19 cases before and after July 3, 2020 in counties implementing a mask mandate (n = 24) relative to those without such mandates (n = 81).
Summary of Main Findings
Following the implementation of mask mandates, the seven-day moving average of COVID-19 incidence decreased by 6% in counties adopting mask mandates but doubled (100% increase) in counties opting out of these mask mandates. Approximately two-thirds (67%) of the state’s population resided in counties implementing mask mandates, which were dispersed throughout Kansas but highly clustered.
The investigators used natural experiment methods (i.e., difference-in-difference estimation) to compare rates of COVID-19 before and after implementation of mask mandates across Kansas’ 105 counties. These mandates were put in place by the state during a time in which few other state-wide policies and factors were influencing the spread of COVID-19 differently between opt-in and opt-out counties. This makes it more plausible, but not certain, that a substantial proportion of the differences in COVID-19 cases between mask mandate opt-in vs opt-out counties were driven by the mask mandates and subsequently mask-wearing behavior. The authors also attempted to account for differences between city-mandated mask orders, some of which were nested in counties opting out of the state’s mask mandate, by reanalyzing their data, which produced similar (but smaller) effect size estimates compared to their published analysis.
Because the authors compare COVID-19 incidence in counties with and without mask mandates, county-level differences in mask-wearing behaviors, mask ordinance implementation, and ordinance enforcement limit attribution of incidence reductions to policy implementation alone. Counties introducing a mask mandate would also be more likely to impose other restrictions (e.g., restaurant/business occupancy restrictions, suspension of mass gatherings) that would likely affect COVID-19 transmission dynamics. In fact, over half (54%) of counties with a mask mandate introduced at least one other COVID-19 mitigation strategy, while fewer than 10% of counties without a mask mandate implemented such control measures. In addition, other unmeasured factors, like behavior change in response to rising COVID-19 incidence, could have different impacts on COVID-19 incidence in opt-in versus opt-out counties. This limits the degree to which COVID-19 incidence differences can be attributed to the effectiveness of mask mandates.
This study is among the largest studies in the US to compare COVID-19 incidence in counties implementing a mask mandate at a single point in time to those opting out of a mask mandate, and is implemented in a scenario that makes assumptions more plausible than many other mask mandate policy studies.
This review was posted on: 19 December 2020