This is a Phase 4 randomized, observer-blind, parallel-controlled study, for evaluation of safety and immunogenicity of sequential immunization of a recombinant SARS-CoV-2 vaccine (adenovirus type 5 vector) in Chinese healthy adults aged 18-59 years after the priming vaccination of inactivated vaccine.

300 healthy subjects aged 18-59 years will be recruited in this study. Of them, 200 subjects who have been vaccinated with two dose 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.