Association of County-Wide Mask Ordinances with Reductions in Daily CoVID-19 Incident Case Growth in a Midwestern Region Over 12 Weeks
Authors: Shacham Enbal , Scroggins Stephen , Ellis Matthew , Garza Alexander 
Published: October 2020

AbstractImportanceThis study assessed the longitudinal impact of new COVID-19 cases when a mask ordinance was implemented in 2 of a 5-county Midwestern U.S. metropolitan region over a 3-month period of time. Reduction in case growth was significant and reduced infection inequities by race and population density.ObjectiveThe objective of this study was to assess the impact that a mandatory mask wearing requirement had on the rate of COVID-19 infections by comparing counties with a mandatory policy with those neighboring counties without a mandatory masking policy.DesignThis was a quasi-experimental longitudinal study conducted over the period of June 12-September 25, 2020.SettingThis study was a population-based study. Data were abstracted from local health department reports of COVID-19 cases.ParticipantsRaw cases reported to the county health departments and abstracted for this study; census-level data were synthesized to address county-level population, income and race.Intervention(s) (for clinical trials) or Exposure(s) (for observational studies)The essential features of this intervention was an instituted mask mandate that occurred in St. Louis City and St. Louis County over a 12 week period.Main Outcome(s) and Measure(s)The primary study outcome measurement was daily COVID-19 infection growth rate. The mask mandate was hypothesized to lower daily infection growth rate.ResultsOver the 15-week period, the average daily percent growth of reported COVID-19 cases across all five counties was 1.81% (±1.62%). The average daily percent growth in incident COVID-19 cases was similar between M+ and M- counties in the 3 weeks prior to implementation of mandatory mask policies (0.90% [±0.68] vs. 1.27% [±1.23%], respectively, p=0.269). Crude modeling with a difference-in-difference indicator showed that after 3 weeks of mask mandate implementation, M+ counties had a daily percent COVID-19 growth rate that was 1.32 times lower, or a 32% decrease. At 12 weeks post-mask policy implementation, the average daily COVID-19 case growth among M- was 2.42% (±1.92), and was significantly higher than the average daily COVID case growth among M+ counties (1.36% (±0.96%)) (p<0.001). A significant negative association was identified among counties between percent growth of COVID-19 cases and percent racial minorities per county (p<0.001), as well as population density (p<0.001).Conclusions and RelevanceThese data demonstrate that county-level mask mandates were associated with significantly lower incident COVID-19 case growth over time, compared to neighboring counties that did not implement a mask mandate. The results highlight the swiftness of how a mask ordinance can impact the trajectory of infection rate growth. Another notable finding was that following implementation of mask mandates, the disparity of infection rate by race and population density was no longer significant, suggesting that regional-level policies can not only slow the spread of COVID-19, but simultaneously create more equal environment.Key PointsQuestionHow are local mask ordinances associated with growth of COVID-19 cases among adjacent counties?FindingsEcological longitudinal analysis reveals a significant slowing of daily COVID-19 case growth after mask ordinance implementation among counties.MeaningLocal-level policy of mask ordinances are shown to be an effective COVID-19 mitigation strategy even within locations of diverse populations.