To evaluate immune response and safety of a bivalent mRNA booster (ancestral/BA.4/5) vaccine in individuals who had received inactivated COVID-19 vaccine with different heterologous boost regimens. A prospective open-label study of bivalent ancestral/Omicron BA.4/5 was conducted. Healthy participants (age > 18) who completed two doses of inactivated COVID-19 vaccine and received any of these booster vaccines (Ad26.COV2.S, ChAdOx1, or mRNA-based) at least 12 months prior were enrolled. Immunogenicity data (anti-Spike (S) IgG, neutralization antibody titer (NT) against ancestral and XBB.1.5, and S-specific IFN-γ T- cells) was obtained at baseline, Day 28±7, and Day 90±14. Of 190 participants enrolled; 57 received Ad26.COV2.S, 66 received ChAdOx1, and 67 received mRNA vaccine as the third dose, respectively. Following bivalent mRNA vaccination, anti-S IgG rose at Day 28, and declined at Day 90. In contrast, the NTtiters against ancestral peaked at Day90. The NTagainst XBB.1.5 peaked at Day 28 with the highest fold rise in the mRNA vaccine subgroup (29.16 [19.55-43.49]), followed by the ChAdOx1 (20.94 [14.18-30.92]), and the Ad26.COV2.S subgroups (13.04 [8.61-19.74]). Geometric concentration of T-cells producing IFN-γ rose comparably in all three subgroups. Bivalent mRNA ancestral/BA.4/5 vaccine enhanced humoral immunity against both ancestral and Omicron XBB1.5, and T-cell immunity in inactivated COVID-19 vaccine primed with different heterologous boost participants. The study was registered at WHO platform: Thai Clinical Trial Registry (TCTR20230811004). 50 50 50
