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

Abstract

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 Québec 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 Québec 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 Québec. Our data also confirm that many infections escaped detection, including a substantial proportion that were asymptomatic.

Seroprevalence of SARS-CoV-2 antibodies among blood donors in Québec: an update from a serial cross-sectional study

“…Compared with the cumulative incidence rate reported by public health authorities (i.e., 3.3% as of March 11, 2021), the estimate in the unvaccinated population suggests that 2.4 COVID-19 cases may go undetected for every confirmed case reported to public health authorities. This ratio of undiagnosed to diagnosed cases (i.e., 2.4:1) is largely similar to that observed in our previous assessment of seroprevalence in May and June 2020 (2.2% vs. 0.6%; a ~ 3:1 ratio) (Lewin et al, 2021 ). Taken together, and consistent with other published results (Byambasuren et al, 2021 ), our data suggest that a substantial proportion of SARS-CoV-2 infections remain undetected, despite notable improvements in access to testing since the early phase of the pandemic.…”

Seroprevalence of SARS-CoV-2 Antibodies Among Children in School and Day Care in Montreal, Canada

“…20 , 21 Our estimated seroprevalence among children was comparable with seroprevalence among Montreal’s adult population for the first wave (3.2% vs 3.1%) but was lower for the second wave (8.4% vs 13.8%). 22 In other parts of Canada, seroprevalence estimates for children from the first and second waves have been considerably lower, ranging from 0.3% to 1.6%. 23 , 24 Internationally, seroprevalence estimates for children and youth from the first and second waves are quite varied, ranging from 0.6% in Germany, 2 4.3% in France, 10 2.4% to 22.3% in Switzerland, 3 , 11 and 2.4% to 11.2% in the UK, 25 , 26 although the trend of increasing seroprevalence over time was consistently found in the cohort studies.…”

COVID-19 Seroprevalence in Canada Modelling Waning and Boosting COVID-19 Immunity in Canada a Canadian Immunization Research Network Study

“…For interest, in Figures 4, S3 and S6 (assuming increases in κ values of 20%, no change, and 38%, respectively), we plot the corresponding distribution of protection against the virus of the different types of immunity in the population, over 10-year age groups, including no protection (S 1 ), some protection (S 2 + V 1 1 ), a higher level of protection (S 3 + V 1 2 ), and full protection (S 4…”

“…A required step in understanding possibilities of resurgence or future vaccine needs lies in the determination and quantification of immunity in the Canadian population. Seroprevalence studies in different population cohorts have been conducted (see [1,2] for examples, and [3] for more information, many others are underway-see [4] for details) which can inform immunity distribution calculations. A recent statistical study by the COVID-19 Immunity Task Force (CITF), which incorporates different population seroprevalence studies into their analysis, estimated that the Canadian population has some immunity against COVID-19: due to infection, 5.4% (95% CrI: 0.6 to 15.8) as of 31 May 2021; due to infection and vaccination, 44.9% (95% CrI: 44.2 to 45.8) as of same date [5] (Credible interval (CrI)).…”