Estimating SARS-CoV-2 Seroprevalence in Canadian Blood Donors, April 2020 to March 2021: Improving Accuracy with Multiple Assays

Abstract

We have previously used composite reference standards and latent class analysis (LCA) to evaluate the performance of laboratory assays in the presence of tarnished gold standards. Here, we apply these techniques to repeated, cross-sectional study of Canadian blood donors, whose sera underwent parallel testing with four separate SARS-CoV-2 antibody assays. We designed a repeated cross-sectional design with random cross-sectional sampling of all available retention samples (n = 1500/month) for a 12 -month period from April 2020 until March 2021. Each sample was evaluated for SARS-CoV-2 IgG antibodies using four assays an Abbott Architect assay targeting the nucleocapsid antigen (Abbott-NP, Abbott, Chicago IL) and three in-house IgG ELISAs recognizing distinct recombinant viral antigens: full-length spike glycoprotein (Spike), spike glycoprotein receptor binding domain (RBD) and nucleocapsid (NP). We used two analytic approaches to estimate SAR-CoV-2 seroprevalence: a composite reference standard and LCA. Using LCA to estimate true seropositivity status based on the results of the four antibody tests, we estimated that seroprevalence increased from 0.8% (95% CI: 0.5-1.4%) in April 2020 to 6.3% (95% CI: 5.1-7.6%) in March 2021. Our study provides further support for the use of LCA in upcoming public health crises, epidemics, and pandemics when a gold standard assay may not be available or identifiable. IMPORTANCE Here, we describe an approach to estimating seroprevalence in a low prevalence setting when multiple assays are available and yet no known gold standard exists. Because serological studies identify cases through both diagnostic testing and surveillance, and otherwise silent, unrecognized infections, serological data can be used to estimate the true infection fatality ratio of a disease. However, seroprevalence studies rely on assays with imperfect sensitivity and specificity. Seroreversion (loss of antibody response) also occurs over time, and with the advent of vaccination, distinction of antibody response resulting from vaccination as opposed to antibody response due to infection has posed an additional challenge. Our approach indicates that seroprevalence on Canadian blood donors by the end of March 2021was less than 10%. Our study supports the use of latent class analysis in upcoming public health crises, epidemics, and pandemics when a gold standard assay may not be available or identifiable.

Keywords: IgG; SARS-CoV-2 antibody; latent class analysis; nucleocapsid; receptor binding domain; spike.

FIG 1

Estimated seroprevalence over time. Estimates were derived using two different approaches, composite reference standard and LCA, as described in the methods. Circles represent mean values and bars indicate the 95% confidence intervals. For comparison, cumulative incidence of laboratory-confirmed SARS-CoV-2 infections in the general population for all provinces excluding Quebec over the study period is shown by the gray shaded area.

FIG 2

Regional trends in seroprevalence. Seroprevalence estimates are shown for each of the two profiles identified by LCA and the overall results. Circles represent mean values and bars indicate the 95% confidence intervals.

FIG 3

Seropositivity estimates over time based on predefined assay thresholds. (A) Percent of study participants positive for each assay, based on month of sample collection. (B) Percent of study participants positive for two or more assays. The different assay combinations are indicated in the legend and details are provided in the methods.

FIG 4

Seroprevalence estimates by latent class grouping. Seropositivity was determined using LCA, with two different classes identified among presumptive seropositive samples. Profile 1 was associated with antibodies to NP, Spike, and RBD, while Profile 2 was associated with antibodies to Spike and RBD.

FIG 5

Antibody profiles of vaccinated participants by time since vaccination at time of sample collection. Seropositivity was determined using LCA, with two different classes identified among presumptive seropositive samples. As in Fig. 4, Profile 1 was associated with antibodies to NP, Spike, and RBD, while Profile 2 was associated with antibodies to Spike and RBD.