This is a 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.

Inclusion Criteria:

• Health subjects aged 18-59 years, who have been completed two-dose regimen of inactive SARS-CoV-2 vaccine in the past 3-6 months, or received one dose of inactive SARS-CoV-2 vaccine in the past 1-3 months.
• The subject can provide with informed consent and sign informed consent form (ICF).
• The subjects are able to and willing to comply with the requirements of the clinical trial program and could complete the 6-month follow-up of the study.
• Axillary temperature ≤ 37.0℃.
• Individuals who are in good health condition at the time of entry into the trial as determined by medical history, physical examination and clinical judgment of the investigator and meet the requirements of immunization.

Exclusion Criteria:

• have the medical history or family history of convulsion, epilepsy, encephalopathy and psychosis.
• be allergic to any component of the research vaccines, or used to have a history of hypersensitivity or serious reactions to vaccination.
• women with positive urine pregnancy test, pregnant or breast-feeding, or have a pregnancy plan within six months.
• have acute febrile diseases and infectious diseases.
• have severe chronic diseases or condition in progress cannot be controlled.
• congenital or acquired angioedema / neuroedema
• have the history of urticaria 1 year before receiving the investigational vaccine.
• have asplenia or functional asplenia.
• have thrombocytopenia or other coagulation disorders (which may cause contraindications for intramuscular injection).
• have needle sickness.
• have the history of immunosuppressive therapy, anti-allergy therapy, cytotoxic therapy or inhaled corticosteroids (excluding corticosteroid spray therapy for allergic rhinitis, and acute corticosteroid therapy without dermatitis) over the past 6 months.
• have received blood products within 4 months before injection of investigational vaccines.
• under anti-tuberculosis treatment.
• not be able to follow the protocol, or not be able to understand the informed consent according to the researcher's judgment, due to various medical, psychological, social or other conditions.

Unblinded staff will responsible for the vaccine administration and management, and other investigators will be kept blinded.