Sequential Immunization of Two Doses of Inactivated COVID-19 Vaccine (Omicron) in Vaccinated Population Aged 18 Years and Above

This trial adopts a randomized, double-blind and positive control design, it is planned to recruit 1800 healthy participants who have been vaccinated with 2/3 doses of COVID-19 inactivated vaccine or mRNA vaccine for 3 months. The participants will be divided into two strata according to the types of vaccines administered, including 900 participants of COVID-19 inactivated vaccine and 900 participants of mRNA vaccine. According to the ratio of 1:1:1, each stratum was randomly assigned to three groups: sequential BIBP- COVID-19 inactivated vaccine (Omicron), WIBP-COVID-19 inactivated vaccine (Omicron) or COVID-19 inactivated vaccine (prototype strain). And according to D0, D28 immunization schedule, two doses of corresponding group vaccines are sequentially administered.