Evaluate the Safety and Immunogenicity of Ad5 COVID-19 Vaccines for Booster Use in Children Aged 6-17 Years.

Inclusion Criteria: * Participants aged 6-17 years at the time of enrollment. * Obtain written informed assent from participants and consent from parents, guardians or legal representatives. * Able and willing to complete all the scheduled study procedures during the whole study follow-up period of 12 months. * Have not received any COVID-19 vaccines (for primary groups only). Exclusion Criteria: * Medical or family history of seizures, 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 asthma. * History of vaccine related SAEs after receiving any COVID-19 vaccines. * Positive urine pregnancy test result, females with child bearing potential (have had menarche). * Acute febrile diseases and infectious diseases, medical history of SARS (SARS-CoV-1). * Axillary temperature \>37.0#. * Serious cardiovascular diseases, such as arrhythmia, conduction block, myocardial infarction, severe hypertension and uncontrollable by medications. * 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 aspleenia. * Thrombocytopenia or other coagulation disorders (may cause contraindications for intramuscular injection). * Trypanophobia. * Severe nasal or oral diseases, such as acute rhinitis (sinusitis), allergic rhinitis, oral ulcers, throat redness, and swelling. * Lung function abnormalities such as chronic obstructive pulmonary disease and pulmonary fibrosis. * Abnormal laboratory test indexes that are clinical significant as judged by the investigator (including white blood cell count, lymphocyte count, eosinophils, neutrophils, platelets, hemoglobin, alanine aminotransferase ALT, aspartate aminotransferase AST, total bilirubin , creatinine, activated partial thromboplastin time) (for sentinel and safety groups only). * Respiratory rate ≥ 17 times per minute (for sentinel and safety groups only). * History of receiving immunosuppressant therapy, anti-allergic therapy, cytotoxic therapy, nebulized corticosteroid therapy in the past 6 months (not including corticosteroid spray treatment for allergic rhinitis, and surface corticosteroid treatment for acute non-complicated dermatitis). * Prior administration of blood products in last 4 months. * Received other investigational drugs within 1 month before the study. * Prior administration of live attenuated vaccines within 1 month before the study. * Prior administration of subunit or inactivated vaccines within 14 days before the study. * Current anti-tuberculosis therapy. * Medical history of Covid-19 disease/infection. * History of epidemiological exposure to COVID-19; have traveled to medium or high-risk areas in the past 21 days or have a history of travelling outside the country * 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 second dose: * Newly emerged situations that meet the first-dose exclusion criteria. * Vaccine related SAE post first dose vaccination. * Serious allergic reactions post first dose vaccination. * Other reasons in the opinion of the investigator.