Immunogenicity assessment
For all participants, serum samples were collected on the day of booster dose (before vaccination) and at 28 days after vaccination. For immunogenicity subgroup participants, extra blood samples at 7 and 14 days after vaccination were collected to explore whether booster vaccination could induce earlier immune response. Previous back-up serum samples at 28 days after two initial doses of CoronaVac were also collected (the historical control group), in adults aged 26-45 years old from the previous lot-to-lot consistency study (There were two primary objectives. One of them was to evaluate the superiority of neutralizing antibody response against Omicron BA.1 at 28 days after Omicron vaccine booster as compared with CoronaVac booster, based on the geometric mean titers’ (GMT) ratio and on the seroconversion rate difference. It is worth noting that the 3C control group was used for either 2C or 3C trial group according to the requirement of NMPA (2/3C+1O vs. 3C+1C). The other one was to evaluate the non-inferiority of neutralizing antibody response against Omicron BA.1 at 28 days after one booster dose of Omicron vaccine, compared with the immune response against ancestral strain at 28 days after two initial doses of CoronaVac, based on GMT ratio (2/3C+1O vs. 2C+0).