摘要 : Summary What is already known about this topic?Numerous ecological and laboratory studies suggest face masks are an effective non-pharmaceutical intervention for reducing the spread of coronavirus disease 2019(COVID-19),but cannot... 展开 Summary What is already known about this topic?Numerous ecological and laboratory studies suggest face masks are an effective non-pharmaceutical intervention for reducing the spread of coronavirus disease 2019(COVID-19),but cannot otherwise assess individual-level effects.What is added by this report?Using a prospective cohort of individuals enrolled in a participatory,syndromic surveillance tool prior to the first case of COVID-19 in the United States,we present a novel longitudinal assessment of the effectiveness of face masks. 收起