SARS‐CoV‐2 immunoassays in a predominantly vaccinated population: Performances and qualitative agreements obtained with two analytical approaches and four immunoassays

Background and Objectives Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) serosurveys are typically analysed by applying a fixed threshold for seropositivity (‘conventional approach’). However, this approach underestimates the seroprevalence of anti‐nucleocapsid (N) in vaccinated individuals—who often exhibit a difficult‐to‐detect anti‐N response. This limitation is compounded by delays between the onset of infection and sample collection. To address this issue, we compared the performance of four immunoassays using a new analytical approach (‘ratio‐based approach’), which determines seropositivity based on an increase in anti‐N levels. Materials and Methods Two groups of plasma donors and four immunoassays (Elecsys total anti‐N, VITROS total anti‐N, Architect anti‐N Immunoglobulin G (IgG) and in‐house total anti‐N) were evaluated. First‐group donors (N = 145) had one positive SARS‐CoV‐2 polymerase chain reaction (PCR) test result and had made two plasma donations, including one before and one after the PCR test (median = 27 days post‐PCR). Second‐group donors (N = 100) had made two plasma donations early in the Omicron wave. Results Among first‐group donors (97.9% vaccinated), sensitivity estimates ranged from 60.0% to 89.0% with the conventional approach, compared with 94.5% to 98.6% with the ratio‐based approach. Among second‐group donors, Fleiss's κ ranged from 0.56 to 0.83 with the conventional approach, compared with 0.90 to 1.00 with the ratio‐based approach. Conclusion With the conventional approach, the sensitivity of four immunoassays—measured in a predominantly vaccinated population based on samples collected ~1 month after a positive test result—fell below regulatory agencies requirement of ≥95%. The ratio‐based approach significantly improved the sensitivities and qualitative agreement among immunoassays, to the point where all would meet this requirement.