Decrease in Hospitalizations for COVID-19 after Mask Mandates in 1083 U.S. Counties

Adjodah, D.; Dinakar, K.; Fraiberger, S. P.; Rutherford, G. W.; Glidden, D. V.; Gandhi, M.

Importance: Population-wide facial masking decreases COVID-19 transmission but may also decrease the severity of disease by reducing the viral inoculum to which the wearer is exposed. The mortality of COVID-19 infection decreased in the U.S. in the second wave over the summer of 2020 compared to the first, but reasons for declining severity of disease have not been fully elucidated. Objective: To determine if facial mask mandates instituted in U.S. counties over the spring and summer of 2020 were associated with declining severity of infection as measured by the number of hospitalizations for COVID-19. Design: Data on hospitalizations due to COVID-19; testing access determined by number of tests performed per day per 100,000 people; new cases per day normalized by population; measures of population mobility to control for other non-pharmaceutical interventions such as lockdowns, social distancing, and business closures; age categories in each census tract; and dates of masking mandates in U.S. counties were all obtained from open-sourced epidemiologic datasets. We used a staggered difference-in-difference study design to assess the impact of the introduction of mask mandates (defined as the treatment) on the proportion of hospitalizations due to COVID-19 per week from March 10-September 16, 2020. Setting: U.S. counties with available full datasets on relevant COVID-19 metrics Exposure: Mask mandates Main outcome: Proportion of hospitalizations due to COVID-19 Results: Using data from 1083 counties (34% of U.S. counties, 82% of U.S. population) from 49 states, we found a statistically significant drop in hospitalization rates due to COVID-19 up to 12 weeks following county mask mandates of 7.13 (95% CI: -4.19, -10.1) percentage points, after controlling for age categories by county, testing access, numbers of cases, and population mobility. Conclusion and Relevance: Facial masking may decrease COVID-19 severity by decreasing the viral inoculum to which individuals are exposed. Mask mandates across 1083 counties in the U.S. in 49 states decreased hospitalization rates from COVID-19 even when controlling for other factors that could impact disease severity, including age, testing access, number of cases, and mobility (as a proxy for other non-Pharmaceutical interventions such as sheltering-in-place). This study adds to the growing evidence for the impact of masking on disease severity and on the utility of population-wide facial masking for COVID-19 pandemic control.