COVID Symptoms, Seroprevalence, and Mortality During the First Wave of SARS-CoV-2 in Canada

Background: Canada had substantial mortality from COVID in the first SARS-CoV-2 wave, mostly in nursing homes. Efforts to stem the pandemic (including vaccination, now being introduced) can benefit from direct understanding of the relationship of age-specific mortality to SARS-CoV-2 seroprevalence and from information on asymptomatic infection. Methods: The Action to Beat Coronavirus (Ab-C) study surveyed a reasonably representative sample of 19,994 adult Canadians about COVID symptoms and analyzed IgG antibodies against SARS-CoV-2 from self-collected dried blood spots (DBS) in 8,967 adults. A sensitive and specific chemiluminescence ELISA detected IgG to the spike trimer. We compared seroprevalence to deaths to establish infection fatality rates (IFRs) and used mortality data to estimate infection levels in nursing home residents. Findings: The best estimate (high specificity) of adult seroprevalence nationally is 1.7%, but as high as 3.5% (high sensitivity) depending on assay cutoffs. The highest prevalence was in Ontario (2.4-3.9%), the most populous province, and in younger adults aged 18-39 years (2.5-4.4%). Based on mortality, we estimated 13-16% of nursing home residents became infected. The first viral wave infected 0.54-1.08 million adult Canadians, half of whom were 70 times higher than comparably-aged adults living in the community. Seropositivity correlated with COVID symptoms, particularly during March. Asymptomatic adults constituted about a quarter of definite seropositives, with a greater proportion in the elderly. Interpretation: Canada had relatively low infection prevalence and low IFRs in the community, but not in nursing homes, during the first viral wave. The Ab-C study demonstrates the practicability of using self-collected DBS for antibody testing in national surveys to monitor the pandemic and vaccine-induced immunity in the population. Funding Statement: Pfizer Global Medical, Unity Health Foundation, Canadian COVID-19 Immunity Task Force Declaration of Interests: We declare no competing interests. Ethics Approval Statement: Ethics approval was provided by St. Michael's Institutional Review Board. REB 20-107