Inclusion Criteria:  Healthy adults 18 years of age and above at the time of enrollment; Able to provide consent to participate in and having signed an Informed Consent Form (ICF); Able and willing to complete all the scheduled study procedures during the whole study follow-up period; HIV negative; No nasal or oral diseases, such as acute rhinitis (sinusitis), allergic rhinitis, oral ulcers, sore throat, etc; IgG ang IgM negative for Covid-19; Axillary temperature ≤37.0℃; No contact history of Covid-19.  Exclusion Criteria:  Serious cardiovascular diseases, such as arrhythmia, conduction block, myocardial infarction, severe hypertension and uncontrollable medications (on-site measurement: systolic blood pressure ≥160mmHg, diastolic blood pressure ≥100mmHg), etc; Abnormal laboratory test indicators which investigator decide to be clinically significant; (Only Phase I volunteers) Respiratory rate ≥17 per minute; Abnormal lung function such as asthma, chronic obstructive pulmonary disease, pulmonary fibrosis, etc; Prior Covid-19 vaccinations; Symptoms of upper respiratory track infections; Medical or family history of convulsions, epilepsy, encephalopathy, and psychosis disorders; History of allergies to any ingredient of Ad5-nCoV, history of serious allergic reactions to any vaccine, history of allergies and immune diseases; Acute febrile diseases and infectious diseases; Medical history of SARS (SARS-CoV-1); Severe chronic diseases or with advanced stage conditions which cannot be controlled smoothly, such as diabetes, thyroid disease, etc; Congenital or acquired angioedema/neurological edema; Urticaria history within 1 year before receiving the study vaccine; Asplenia or functional asplenia; Thrombocytopenia or other coagulation disorders (may cause contraindications for intramuscular injection); Trypanophobia in intramuscular injection groups; History of receiving immunosuppressant therapy (continuous oral or instillation for more than 14 days), anti-allergic therapy, cytotoxic therapy, nebulized corticosteroid therapy in the past 6 months; Prior administration of blood products in last 4 months; Other vaccination(s) or investigational drugs within 1 month before study onset, or planned use during the study period; Prior administration of live attenuated vaccine within 1 month before study onset; Prior administration of subunit or inactivated vaccine within 14 days before study onset; Current anti-tuberculosis therapy; Women who are pregnant or lactating, positive urine pregnancy test or plan to become pregnant during the whole study; Any condition that in the opinion of the investigators may interfere with the participants' compliance or evaluation of study objectives or informed consent (i.e. medical, psychological, social or other conditions, etc.) Exclusion criteria for the phase II portion of the study will be detailed in an amended synopsis/study protocol.