Strong effect of Anti-COVID-19 vaccination on the peak titer of antibodies against novel coronavirus: factors affecting humoral immunity in Georgian plasma donors

OBJECTIVE: Humoral responses to COVID-19 vaccination vary across populations, and evidence on the role of ABO and the Rhesus factor remains inconsistent. This prospective cohort study examined peak antibody responses among 175 voluntary plasma donors in Georgia during 2022-2023, a setting where the prevalence of Rhesus negativity is relatively high and no prior data have been reported. RESULTS: Using quantitative anti-spike IgG ELISA, we measured peak antibody titers over a six-month period following vaccination and/or natural infection. Vaccination emerged as the strongest predictor of high antibody response (≥ 1000 BAU/mL), with 81.9% of vaccinated versus 21.0% of unvaccinated participants achieving protective levels (OR = 16.88, p < 0.001). Rhesus-positive individuals were significantly more likely to achieve protective titers than Rhesus-negative (57.7% vs. 30.8%, p = 0.011), while ABO blood group showed no association with antibody response (p = 0.374). Hybrid immunity provided optimal protection, with 92.5% of participants achieving elevated titers. These findings highlight vaccination and hybrid immunity as central determinants of humoral response, while identifying the Rhesus factor as an additional contributor. This study represents the first dataset of its kind from the Caucasus region, providing distinct regional evidence with implications for global immunological research.