LAUSR

Purpose Ecological and laboratory studies suggest face masks are an effective non-pharmaceutical intervention for reducing spread of SARS-CoV-2. These studies cannot measure individual risk reduction or account for individual behavioral and demographic confounders. Here we present a novel longitudinal assessment of the protective role of masks in a national cohort of individuals enrolled in a syndromic surveillance tool prior to the first case of COVID-19 in the United States. Methods & Materials The study population consisted of a subset of participants (N=4,723 adults) enrolled in Flu Near You (FNY), a web-based longitudinal syndromic surveillance platform. Weekly self-reports of respiratory syndromes were used to assess the onset of COVID-like illness (CLI) symptoms from January to June 2020. An annual retrospective questionnaire submitted by this subset of FNY participants assessed precautionary behaviors (masking, distancing, etc.) and demographic information. We used a previously validated exposure variable (self-reported likelihood to wear masks while visiting family and friends and while grocery shopping) to measure mask wearing. A Cox proportional hazards model was used to assess the effect of mask wearing on CLI while controlling for age, gender, precautionary behavior (social distancing contacts, adoption date), county population density and time-varying county COVID-19 burden. Results There were 1,293 reports of respiratory symptoms over the study period. Individuals characterized as most likely to wear masks were 45% [24%-61%] less likely to report symptoms of COVID-like illness compared to individuals characterized as least likely to wear masks. Mask-wearing also demonstrated a protective effect for those characterized as somewhat likely to wear masks (HR: 0.60, 95% CI: 0.42-0.84, p=0.003) and those who were likely to wear masks in only one of the two circumstances (HR. 0.59, 95% CI: 0.42-0.83, p=0.002), compared to respondents least likely to wear masks. Sensitivity analyses with alternative broad and narrow CLI definitions produced a similar magnitude and protective effect. Conclusion Face masks were effective as a non-pharmaceutical intervention at preventing respiratory illness in the FNY population. The individual risk reduction was consistent with previous ecological measures of the protective effect of face masks, as well as robust to adjustment for behavioral, demographic, and environmental confounders.