Discussion
infection
INFECTION, DISEASE
In this trial, the booster dose of Omicron-adapted vaccine elicited superior neutralizing antibody responses against Omicron BA.1 compared to CoronaVac booster, which was demonstrated by the immunogenicity endpoints of GMT ratio and seroconversion rate difference following the guidelines of regulatory authorities. These results were comparable to other Omicron vaccines. Pfizer-BioNTech and Moderna have each developed two Omicron-containing vaccines, which are bivalent mRNA vaccines of ancestral and Omicron BA.1 as well as ancestral and Omicron BA.4/BA.5. In their clinical trials, the second booster dose of bivalent vaccines was administrated to participants who had received three prior doses of prototype vaccines (The setting of a non-inferiority criterion based on humoral immune response levels could be an alternative approach to extrapolate the efficacy of Omicron vaccine as compared with the immune response of two initial doses of CoronaVac, for which the efficacy had been previously documented (Additionally, in our study, the antibody response against Omicron BA.1 after Omicron vaccine booster was much weaker than that against ancestral strain, with GMTs of 9.30 vs. 56.51 and 10.36 vs. 84.41 in 2C and 3C cohorts, respectively. Similarly, for Pfizer-BioNTech’s monovalent BA.1 vaccine booster, the GMT against Omicron BA.1 and ancestral strain was 1015 and 5539, respectively (Another concern is that Omicron is mutating further, perhaps leading to easier breaking of current vaccine protection. In our study, we observed that Omicron vaccine booster induced much lower neutralizing antibody titers to Omicron BA.5 as compared with against Omicron BA.1. Similar trends were also found in Pfizer-BioNTech and Moderna’s bivalent (ancestral and Omicron BA.1) booster vaccines, of which the GMTs against Omicron BA.4/5 and BA.1 were 114 vs. 711 and 727 vs. 2372, respectively (Breaking immune imprinting has become the key issue for new variant vaccine development. Given the waning humoral and cellular immune responses after prior infection or vaccination in previous months (Limitations of this trial included that only humoral immune response against Omicron variants was assessed, whereas cellular immune response also contributes to protection against infection and severe disease (In conclusion, a booster dose of Omicron-adapted vaccine is well-tolerated and can elicit a superior immune response against Omicron BA.1 as compared with CoronaVac; however, the humoral immune response induced is weak.