This is a phase II, randomised, double-blinded and placebo-controlled clinical trial in healthy adults above 18 years of age,inclusive, who meet all eligibility criteria.

This clinical trial is designed to evaluate the immunogenicity and safety of Ad5-nCoV which encodes for a full-length spike (S) protein of SARS-CoV-2. 500 subjects will be enrolled, 250 subjects in middle-dose vaccine group, 125 subjects in low-dose and placebo group, respectively. Immunogenicity will be tested on days 0, 14, 28 and 6 months after vaccination