This is a randomized, observer-blind, parallel-controlled Phase 4 study, for evaluation of safety and immunogenicity of sequential immunization of a recombinant COVID-19 vaccine (adenovirus type 5 vector) in Chinese healthy adults aged 60 and above after the priming vaccination of inactivated vaccine. 300 healthy subjects aged 60 and above will be recruited in this study. Of them, 200 subjects who have been vaccinated with two doses of inactive SARS-CoV-2 vaccine will be recruited and randomized at a 1:1 ratio to receive a booster dose of inactivated SARS-CoV-2 vaccine or recombinant SARS-CoV-2 Ad5 vectored vaccine at 3~6 months later. Other 100 subjects who have been vaccinated with one dose of inactivated SARS-CoV-2 vaccine will be recruited and randomized at a 1:1 ratio to receive a booster dose of inactivated SARS-CoV-2 vaccine or recombinant SARS-CoV-2 Ad5 vectored vaccine at 1~3 months later.

The occurrence of adverse events within 28 days and serious adverse events within 6 months after vaccination will be observed. In addition, blood samples will be collected on day 0 before the boosting with ad5 vectored vaccine and on day 14, 28 and month 6 after the boosting. Serum antibody levels, cellular immune responses and the subgroups or germlines of the specific B cells will be analyzed.

Each subject will remain in this study for approximately 6 months.