Immunogenicity Persistence of a Third-Dose Homologous BBIBP-CorV/CoronaVac Boosting Vaccination: A Prospective Open-Label Study.

The inactivated whole-virion severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine has been widely used in a two-dose schedule, but with insufficient data on the immunogenicity of homologous BBIBP-CorV/CoronaVac boosting vaccination and too little follow-up to assess the duration of the immunogenic response. We prospectively evaluated the immunogenicity of a third-dose BBIBP-CorV/CoronaVac boosting vaccination, with neutralizing titers against wild type and Omicron assessed at the baseline (immediately before the booster dose), and days 14, 28, 98, and 174 post the third-booster. Of 182 volunteers screened, 165 were assessed eligible for enrolment. No moderate/severe adverse events were observed during the term of the study. From the baseline to day 174 post the third booster, neutralizing titers against wild type and Omicron peaked by approximately sixfold increase (up to 811.83 and 33.40, respectively) at day 14 and slowly decreased over time. The geometric mean titers against Omicron were lower than against type with a 19.8-39. Sixfold reduction at all time points. The seropositivity against Omicron at the baseline, days 14, 28, 98, and 174 after the booster dose was 12.6%, 50.0%, 37.8%, 38.6%, and 22.8%, respectively. Data presented herein indicated that the BBIBP-CorV/CoronaVac booster significantly enhances the neutralizing potency against wild-type strain but elicited weaker neutralizing activity to Omicron. Our findings suggest that individuals receiving booster inactivated vaccine remain at risk for Omicron infection, which is crucial to inform ongoing and future vaccination strategies to combat coronavirus disease 2019.