Clinical Trial on Sequential Immunization of Recombinant COVID-19 Vaccine (CHO Cells,NVSI-06-08) and Inactivated COVID-19 Vaccine (Vero Cells) in Population Aged 18 Years and Above

This is randomized, blinded and controlled design. Among the randomly selected subjects who have been vaccinated with two doses of Inactivated COVID-19 vaccine (Vero cell), based on a step-wise approach, the subjects will receive one dose of recombinant COVID-19 vaccine sequentially at different shedules of 4-6 months, 7-9 months and \>9 months after two doses of vaccination, and the subjects vaccinated at different schedules will be randomly assigned to different sequential immunization groups. At the same time, each sequential immunization group will be matched with a control group with the inactivated COVID-19 vaccine (vero cells) as the booster dose.