SARS-CoV-2 seroprevalence among blood donors in Québec, and analysis of symptoms associated with seropositivity: a nested case-control study.

Objectives A substantial proportion of individuals infected with SARS-CoV-2 do not experience noticeable symptoms typical of COVID-19. Our objectives were to evaluate the impact of the first wave of the pandemic in Quebec by measuring SARS-CoV-2 antibody seroprevalence in a convenience sample of healthy blood donors and to study the association between seropositivity and the occurrence of COVID-19 symptoms. Methods The study design was a cross-sectional serological survey with a nested case-control study. Residual blood samples from donations collected between May 25 and July 9, 2020 (well before vaccination rollout) in the province of Quebec were tested for anti-Spike RBD antibodies by ELISA. Seropositive donors and a control group of seronegative donors were questioned about prior COVID-19 symptoms. All qualified blood donors were eligible for participation. Results A total of 7691 blood donors were included in the study. After adjustments, the seroprevalence rate was 2.2% (95% CI 1.9-2.6). Seropositive donors reported one or more symptoms in a proportion of 52.2% (95% CI 44.2-60.1); this proportion was 19.1% (95% CI 13.4-26.1) among seronegative donors, suggesting that approximately 50-66% of all infections were asymptomatic. Univariate analysis of associations between symptoms and seropositivity revealed that except for rhinorrhea, all symptoms were significantly associated with seropositivity. Conclusion Assuming that blood donors are fairly representative of the general adult population, this study shows that less than 3% of 18-69-year-olds have been infected during the first wave of the pandemic in the province of Quebec. Our data also confirm that many infections escaped detection, including a substantial proportion that were asymptomatic.