Fighting Against COVID-19 Requires Wearing a Face Mask by Not Some but All
One sentence summary: Causal effects of social interaction on COVID-19 are statistically eliminated
when more than 85% of people "always" wear a face mask.
The corresponding academic paper by Hakan Yilmazkuday is available as a working paper here.
Abstract
This paper investigates the effects of wearing a face mask on fighting against coronavirus disease 2019 (COVID-19). The formal analysis is achieved by using a difference-in-difference design, where U.S. county-level data on changes in COVID-19 cases or deaths are regressed on lagged changes in social interaction of people measured by Google mobility. The main contribution is achieved by distinguishing between the effects of social interaction on COVID-19 in mask-wearing versus non-mask-wearing counties determined by Dynata surveys. After controlling for county-specific and time-specific factors, the results show that social interaction causally increases both COVID-19 cases and deaths across U.S. counties. Wearing a face mask starts working to fight against COVID-19 only if more than 75% of people in a county "always" wear a face mask, while the effects of social interaction on COVID-19 are statistically eliminated when more than 85% of people in a county "always" wear a face mask.
Non-technical Summary
Social interaction between people is accepted as one of the key determinants for the spread of viruses leading to infections, including coronavirus disease 2019 (COVID-19). Despite its leading effects on COVID-19, social interaction is still necessary to prevent the corresponding societal and economic costs. Accordingly, wearing a face mask in public has been suggested by several studies to be able to continue having social interactions during the COVID-19 era as mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets in both laboratory and clinical contexts.
Despite the consistency in the recommendation that especially symptomatic individuals should use face masks, discrepancies have been observed in the general public and community settings regarding face-mask wearing. Accordingly, this paper investigates the effects of wearing a face mask on the causal relationship between social interaction and COVID-19 cases or deaths. This initially requires confirming the causal relationship between social interaction and COVID-19 cases or deaths. This confirmation is achieved by using daily data from U.S. counties on COVID-19 cases or deaths as well as social interaction measures based on Google mobility for the period between February 15th, 2020 and August 30th, 2020. The formal analysis is achieved by using a difference-in-difference design, where U.S. county-level data on changes in COVID-19 cases or deaths are regressed on lagged changes in social interaction of people after controlling for county-specific and time-specific factors. The results of this initial investigation confirm that higher social interaction leads to higher COVID-19 cases and deaths across U.S. counties.
After confirming the causal relationship between social interaction and COVID-19 cases (or deaths), we continue with a secondary investigation regarding the effects of wearing a face mask on this relationship. In order to do so, we categorize U.S. counties as mask-wearing counties versus non-mask-wearing counties by using Mask-Wearing Survey Data collected by Dynata at the request of New York Times from 250,000 survey respondents. This categorization of U.S. counties results in splitting the effects of social interaction on COVID-19 cases/deaths into those in mask-wearing counties versus non-mask-wearing counties.
The results of this secondary investigation reveal that wearing a face mask starts working to fight against COVID-19 cases or deaths only if more than 75% of people in a county "always" wear a face mask, while the effects of social interaction on COVID-19 are statistically eliminated when more than 85% of people in a county "always" wear a face mask. Therefore, it is possible to continue having social interactions without any statistically significant effects on COVID-19 cases if a community-wide wearing of face masks can be achieved. This result is important as it provides insights about how societal and economic costs due to COVID-19 can be prevented by wearing a face mask by not some but all.
The corresponding academic paper by Hakan Yilmazkuday is available as a working paper here.