Antibody titers and the risk of infection during the SARS-CoV-2 Omicron phase in Bizen City, Japan.

BACKGROUND:Understanding the association between the immune response and the risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has implications for forthcoming prevention strategies. We evaluated the association between antibody titers and the risk of infection for the general population during the Omicron-dominant phase. METHODS:This was a prospective cohort study of residents or people affiliated with institutions in Bizen City, which included 1,899 participants. We measured the titers of antibodies against SARS-CoV-2 repeatedly every 2 months from June 2022 to March 2023. Infection status was obtained from self-reported questionnaires and the official registry. We estimated risk ratios (RRs) for infection within 2 months of the date of each antibody measurement with 95% confidence intervals (CIs) based on antibody titer categories and spline functions. RESULTS:Compared with the <2,500 arbitrary unit (AU)/mL category, the 2,500-5,000, 5,000-10,000, and ≥10,000 AU/mL categories had adjusted RRs (95% CI) of 0.81 (0.61-1.08), 0.51 (0.36-0.72), and 0.41 (0.31-0.54), respectively. The spline function showed a non-linear relationship between antibody titer and risk. CONCLUSIONS:Higher antibody titers were associated with a lower risk. We demonstrate the usefulness of measuring an antibody titers to determine the appropriate timing for future vaccination.