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

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

Inclusion Criteria:

• Aged between 18 and 60 years.
• Able to understand the content of informed consent and willing to sign the informed consent
• Able and willing to complete all the secluded study process during the whole 6 months study follow-up period.
• Negative in HIV diagnostic test.
• Negative in serum antibodies (IgG and IgM) screening of COVID-19.
• Axillary temperature ≤37.0°C.
• The BMI index is 18.5-30.0.
• General good health as established by medical history and physical examination.

Exclusion Criteria:

• Family history of seizure, epilepsy, brain or mental disease
• Subject allergic to any component of the investigational vaccine, or a more severe allergic reaction and history of allergies in the past.
• Woman who is pregnant, breast-feeding or positive in β-HCG (human chorionic gonadotropin) pregnancy test (urine) on day of enrollment, or become pregnant during the next 6 months
• Any acute fever disease or infections.
• History of SARS
• Major congenital defects or not well-controlled chronic illness, such as asthma, diabetes, or thyroid disease.
• Serious cardiovascular diseases, such as arrhythmia, conduction block, myocardial infarction, severe hypertension without controllable drugs, etc.
• Hereditary angioneurotic edema or acquired angioneurotic edema
• Urticaria in last one year
• No spleen or functional spleen.
• Platelet disorder or other bleeding disorder may cause injection contraindication
• Faint at the sight of needles.
• Prior administration of immunodepressant or corticosteroids, antianaphylaxis treatment, cytotoxic treatment in last 6 months.
• Prior administration of blood products in last 4 months
• Prior administration of other research medicines in last 1 month
• Prior administration of attenuated vaccine in last 1 month
• Prior administration of inactivated vaccine in last 14 days
• Current anti-tuberculosis prophylaxis or therapy
• According to the judgement of investigator,various medical, psychological, social or other conditions, those could affect the subjects to sign informed consent.