Difficulty recruiting subjects, so the study was terminated.

This is a multicenter, randomized, and open-label study to evaluate the immune responses and safety profiles of children aged 6-12 years and adolescents aged 13-17 years receiving Ad5-nCoV (intramuscular injection) or Ad5-nCoV-IH (nebulized inhalation) ≥ 90 days after receiving two doses of CoronaVac.

A total of 1000 participants will be equally divided into the two study groups, children of 6-12 years and adolescents of 13-17 years. In each age group, they will have an equal chance to received either one dose of Ad5-nCoV (intramuscular injection) or Ad5-nCoV-IH (nebulized inhalation). The enrollment of the children group will start after the safety profiles in 7 days post-vaccination of the adolescent group have been deemed acceptable. Investigators should try to balance the age and sex of the participants during enrollment.

Inclusion Criteria:

1. Children aged 6-12 years and adolescents aged 13-17 years at the time of randomization.
2. Have received a complete two dose schedule of CoronaVac for ≥ 90 days.
3. Obtain written informed assent from participants and consent from parents, guardians or legal representatives.
4. Subjects are eligible for immunization of this product as evaluated by investigators after medical history examination, physical examination and clinical judgment of health.

Exclusion Criteria:

1. Have a history of seizures, epilepsy, encephalopathy, psychosis.
2. History of anaphylaxis to any vaccine component.
3. Positive urine pregnancy test result, pregnant, lactation women. Female had menarche must conduct the urine pregnancy test.
4. Congenital or acquired angioedema/neuroedema .
5. Medical history of Guillain-Barré syndrome.
6. Have had asthma attacks within 2 years.
7. Have severe nasal or oral diseases, such as rhinitis (sinusitis), allergic rhinitis, oral ulcer, throat swelling, etc.
8. Asplenia or functional absence of spleen.
9. Bleeding disorder (e.g. protein S or factor deficiency, coagulopathy or platelet disorder).
10. Any confirmed or suspected immunosuppressive or immunodeficient state; received immunosuppressive therapy, cytotoxic therapy, or chronic corticosteroids (excluding corticosteroid spray therapy for allergic rhinitis and surface corticosteroid therapy for acute non-complicated dermatitis) within the past 6 months.
11. Have chronic systematic infection or chronic pulmonary disease
12. Administration of immunoglobulins and/or any blood products within three months prior to the planned administration of the vaccine candidate.
13. Receiving anti-tuberculosis or cancer treatment.
14. History of COVID-19 disease.
15. Have a positive result at the examination of rapid SARS-CoV-2 antibody assay (S-RBD IgG) before vaccination.
16. Have received COVID vaccines other than CoronaVac.
17. Received or plan to receive any non-COVID vaccines (licensed or investigational), within 14 days before and after study vaccination.
18. Current diagnosis of or treatment for cancer, e.g. leukemia.
19. Any other significant disease, disorder or finding which may significantly increase the risk to the volunteer because of participation in the study, and affect the ability of the volunteer to participate in the study or impair interpretation of the study data.