Seroprevalence of SARS-CoV-2 antibodies among Japanese healthcare workers from 2020 to 2022 as assayed by two commercial kits

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 2538 participants, seroprevalence was found to be 16.6% 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 at the 2022 health physical. Using PCR positive test date as day 0, while the positivity of the total antibody assay was retained for the entire study period (until more than 24-months post-infection), the IgG-only assay's positivity declined after month 4. The Mantel-Haenszel test found a significant difference in the two assays' seropositivity, between stratified groups of "within 3 months" and "4 months or more" from infection (P < 0.001). Our study found significant differences in seropositivity over time of total antibody versus IgG-only assays, suggesting an optimal assay for retaining sensitivity over the entire infection period when designing seroprevalence studies.