LAUSR

Key Points Question What was the cumulative incidence of SARS-CoV-2 infection during the first 2 viral waves (April to July 2020 and October 2020 to March 2021) among the Canadian adult population? Findings In this cohort study of a representative sample of 19 994 adult Canadians, analyses of serial survey responses and dried blood spots revealed that the cumulative incidence of SARS-CoV-2 infection among unvaccinated adults increased from 1.9% after the first viral wave to 6.5% after the second viral wave. Seropositivity was more demographically and geographically homogeneous during the second wave than the first, and more than 80% of seropositive adults in the first wave who had blood samples retested after the second wave remained seropositive. Meaning This study found that the cumulative incidence of SARS-CoV-2 infection was modest in Canada until March 2021; this incidence was lower than the levels of population immunity required to substantially reduce transmission of the virus.