The trial protocol was not approved by the regulatory authorities.

This phase III clinical trial is an endpoint-driven randomized, double-blind, parallel-controlled, multicenter clinical trial, and around 13,000 subjects aged 18 years and above who have previously received 1 dose of intramuscular Ad5-nCoV will be recruited. Volunteers should have been vaccinated with intramuscular Ad5-nCoV > 56 days prior to enrollment. All subjects will receive 1 dose of investigational vaccine or placebo through nebulized inhalation. The ratio of subjects in the vaccine group and placebo group is 1:1 and the efficacy and safety of the investigational vaccine will be followed up for 52 weeks after vaccination.

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.