Anti-SARS-CoV-2 receptor-binding domain IgG titers and associated factors after heterologous mRNA booster vaccination in individuals primed with the inactivated vaccine.

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), continues to circulate globally, highlighting the need to understand immune responses after heterologous booster vaccination. This cross-sectional study evaluated anti-SARS-CoV-2 receptor-binding domain (RBD) IgG titers and associated factors among adults who had completed a two-dose CoronaVac primary series and received a mRNA-based booster dose (BNT162b2 or mRNA-1273) 1-13 months before sampling, with no documented or self-reported history of COVID-19. Anti-SARS-CoV-2 RBD IgG was measured using the Abbott SARS-CoV-2 IgG II Quant assay. Antibody titers were ln-transformed and analyzed using independent t-tests, one-way analysis of variance, and log-linear regression, with results reported as geometric mean titers (GMTs) and geometric mean ratios (GMRs). A total of 158 participants were included. Anti-SARS-CoV-2 RBD IgG GMTs were not significantly different between the two booster groups. In the combined analysis, the highest GMT was observed in the ≤3-month post-booster interval group, followed by lower GMTs at longer intervals. After adjustment for age, sex, and body mass index, participants sampled at 4-6 months had significantly lower titers than those sampled at ≤3 months (adjusted GMR=0.42; 95%CI: 0.22-0.82). Overweight participants had higher titers than those with normal body mass index in the combined analysis (adjusted GMR=1.73; 95%CI: 1.04-2.86). These findings suggest lower humoral immune responses with increasing time after heterologous mRNA booster vaccination in individuals primed with CoronaVac and may inform future booster strategies.