A Immuno-bridging and Immunization Schedules Study of COVID-19 Vaccine (Vero Cell), Inactivated

This trial is a randomized study to evaluate the immunogenicity of COVID-19 Vaccine (Vero Cell), Inactivated and in population aged 60 years old and 3-17 years old with different schedules (D0,21,42, D0, 21,111,or D0,21,171)compared to that in population aged 18-59 years old with 2-dose schedule (D0,21). And subjects in the same age are randomly assigned to different immunization schedule groups.