Immunogenicity and Safety of Booster Immunization of COVID-19 Vaccine (Vero Cell), Inactivated (Omicron Variant) in Healthy People Aged 18 Years and Above

This is a randomized, double-blind, active-controlled Phase Ⅲ clinical trial, as well as an immuno-bridging clinical trial by parallel testing previous serum after primary immunization of COVID-19 vaccine (Vero cell), inactivated (Omicron variant). The main purpose of this study is to evaluate the superiority of immunogenicity against SARS-CoV-2 Omicron strain induced by one-dose booster immunization of inactivated COVID-19 vaccine (Omicron variant), developed by Sinovac Research and Development Co., Ltd.in subjects who have received two- or three-dose Prototype COVID-19 vaccine （CZ strain）, compared with one-dose booster of Prototype COVID-19 vaccine （CZ strain） in subjects who have received three-dose Prototype COVID-19 vaccine （CZ strain）, and to evaluate the non-inferiority of immunogenicity against SARS-CoV-2 Omicron strain induced by one-dose booster immunization of inactivated COVID-19 vaccine (Omicron variant), developed by Sinovac Research and Development Co., Ltd., in subjects who have received two- or three-dose Prototype COVID-19 vaccine, compared with the immunogenicity against SARS-CoV-2 Prototype strain induced by two-dose Prototype COVID-19 vaccine（CZ strain）after primary immunization.