ABSTRACT
Antibody tests are used as surveillance tools for informing health policy making. However, results may vary by type of antibody assay and timing of sample collection following infection. Long-term longitudinal cohort studies on antibody assay seropositivity have remained limited, especially among Asian populations. Using blood samples obtained at health physicals (2020–2022) of healthcare workers (mass vaccinated with mRNA COVID-19 vaccines) at a Japanese medical center, we measured N-specific antibodies using two commercially available systems. Roche Elecsys® Anti-SARS-CoV-2 measures total antibodies and Abbott AlinityⓇ SARS-CoV-2 IgG measures only IgG. Among 2,538 participants, 16.6% tested positive via total antibody assay versus 12.9% by IgG-only (including grayzone) by mid-June 2022. For 219 cases with a previous PCR-confirmed infection, positivity was 97.3% using total antibody assay versus 76.3% using IgG-only assay. During the first 16 weeks after infection, both assays demonstrated high positivity. However, while positivity of the total antibody assay was retained for the entire study period (until week 113 post-infection), IgG-only assay positivity declined after week 16. Our study found significant differences in the performance of total antibody versus IgG-only assays, suggesting an optimal assay for retaining sensitivity over the entire infection period when designing seroprevalence studies.