Safety and Immunogenicity Study of Inactivated Vaccine for Prevention of SARS-CoV-2 Infection(COVID-19)

This study is a randomized, double-blinded, single-center, placebo-controlled phase 1\&2 clinical trial in healthy elderly aged 60 years and above. The experimental vaccine and placebo were both manufactured by Sinovac Life Sciences Co. , Ltd. A total of 422 subjects will be enrolled, with 72 in phase 1 and 350 in phase 2. 72 Subjects with 36 in medium-dosage group and 36 in high-dosage group in phase 1 will receive two doses of primary immunization according to the immunization schedule of day 0, 28 and the subjects at each dosage group will be assigned in a 2:1 ratio to receive investigational vaccine or placebo respectively.All enrolled subjects will receive 1 dose of booster immunization 1 year after primary immunization.350 Subjects in phase 2 will receive two doses of primary immunization according to the immunization schedule of day 0,28,the subjects will be assigned in a ratio of 2:2:2:1 to receive the low dosage, medium dosage, high dosage vaccine, or placebo. All enrolled subjects will received 1 dose of booster immunization(the third dose ) 6 months after primary immunization.And subjects in medium-dosage group and high -dosage group will receive the second booster dose (the fourth dose) 1 year after the second dose.