Study to Evaluate the inHaled Recombinant COVID-19 Vaccine (Adenovirus Type 5 Vector) On the Protective-Efficacy in Adults (SeiHOPE)

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.