Inclusion Criteria:  Adult volunteers aged 18 years and above who have received 1 dose of intramuscular Ad5-nCoV > 56 days at the enrollment. Volunteers who have provided informed consent and signed the informed consent form. Volunteers who are able and willing to comply with the requirements of the clinical study protocol, and can complete the entire follow-up period of this study.  Exclusion Criteria:  Allergic to the active ingredient, any non-active ingredient, or substances used in the manufacturing process, or developed an allergy to similar vaccines in the past. History of severe anaphylactic reaction to vaccines (e.g., acute anaphylactic reactions, angioedema, dyspnea). History or family history of seizures, epilepsy, encephalopathy, or psychosis. Severe nasal or oral diseases, such as acute rhinitis (sinusitis), allergic rhinitis, oral ulcers, throat redness, and swelling. History of a definite diagnosis of COVID-19. Positive SARS-CoV-2 N antibody rapid screening kit (IgG and IgM) results. Received COVID-19 vaccine other than Ad5-nCoV. Axillary temperature > 37.0°C. Severe cardiovascular diseases, such as arrhythmia, conduction block, myocardial infarction, or severe hypertension and cannot be controlled by drugs. Lung function abnormalities such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis. Acute febrile illness, symptoms of upper respiratory tract infection, or infectious diseases. Severe chronic diseases, or diseases of the progressive stage that cannot be steadily controlled, such as diabetes or thyroid disorders. Asplenia or functional asplenia. Thrombocytopenia or other coagulation disorder. Received immunosuppressant or immunomodulatory therapy (continuous oral or intravenous infusion for more than 14 days), anti-allergic therapy, cytotoxic therapy within the past 6 months. Inhalational and local steroids are allowed. Received blood products within 4 months before receiving the investigational vaccine. Received other vaccines within the past 14 days or plan to receive other vaccines 14 days after scheduled immunization with the investigational vaccine. Volunteers under antituberculous treatment or with active tuberculosis. Received blood/plasma products or immunoglobulin 60 days before immunization with the investigational vaccine or planned to receive blood/plasma products or immunoglobulin during the entire study period. Females who are positive for the urine pregnancy test, pregnant, or breastfeeding, or females who plan to conceive within 3 months (urine pregnancy test will only be performed in female volunteers of childbearing age). Volunteers who are judged by the investigator with any contradiction to the study protocol or affected the signing of informed consent due to various medical, psychological,social, or other conditions.