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Association of County-Wide Mask Ordinances with Reductions in Daily CoVID-19 Incident Case Growth in a Midwestern Region Over 12 Weeks

Our take —

This study, available as a preprint and thus not yet peer reviewed, found that the implementation of a mandatory mask mandate in two metropolitan U.S. counties was significantly associated with a 44% lower COVID-19 case growth in the twelve-week period following the mandate’s introduction. However, because it is likely that other policies and behaviors were occurring concurrently with the policies, it is unclear to what extent these mandates contributed to observed changes in COVID-19 growth rates.

Study design


Study population and setting

The authors used daily confirmed COVID-19 case counts for five counties in the St. Louis, Missouri, metropolitan area over a 15-week period. Case counts were tracked for the three-week period preceding the implementation of a mandatory mask mandate in St. Louis City and St. Louis County (July 3, 2020) and for twelve weeks after. The authors compared daily percent (%) changes in COVID-19 cases prior to and proceeding the implementation of mask mandates, comparing counties with and without mask mandates.

Summary of Main Findings

Daily COVID-19 case growth in the three weeks preceding mask ordinances was similar in counties with and without these mandates. Three weeks after mask mandates were introduced in St. Louis City and St. Louis County, COVID-19 growth rates were 44% lower in jurisdictions with the mask mandates compared to those without mandates. This disparity in COVID-19 case growth continued to widen between counties with and without mask mandates in the 12 weeks following implementation: counties with a mask mandate reported a 78% lower daily case growth rate compared to counties without mandates.

Study Strengths

The authors compare daily COVID-19 case growth in a small sample of geographically proximal counties, some of which enacted mask mandates, and others did not. The comparison of case growth rates before and after these counties helps establish a different pattern of case growth that occurred before and after the mandates were enacted.


Implementation of other non-pharmaceutical measures and behavior change in response to COVID-19 case growth could explain subsequent changes in epidemic trajectories beyond the implementation of the mask mandates. Because policy introduction may not correspond to subsequent behavior change (i.e., increased mask use), the plausibility of a policy introduction alone reducing COVID-19 case growth is questionable. Additionally, the counties included in the analysis may differ, in both measured and unmeasured ways, that could explain changes in daily COVID-19 case growth beyond mask mandates. Moreover, these counties very likely had many social, economic, and political factors, which could have influenced case rates concurrently with mask mandates. Finally, because mask mandates were introduced in these select counties in response to exponential COVID-19 case growth, the measured impact of this policy on COVID-19 transmission may not be consistent if implemented in other settings, including jurisdictions with lower COVID-19 case burdens or non-exponential case growth.

Value added

This is among the first studies to attempt to quantify the impact of a mandatory mask mandate on COVID-19 transmission in metropolitan U.S. counties.

This review was posted on: 5 December 2020