Efficacy, Immunogenicity and Safety of COVID-19 Vaccine , Inactivated in Children and Adolescents

Inclusion Criteria: * Healthy children and adolescents aged 6 months to 17 years; * The participants and/or their guardians are able to understand and sign the informed consent voluntarily (in accordance with the local regulations); * Able to comply with study procedures based on the assessment of the Investigator; * Female participants of childbearing potential (post-menarche girls or in accordance with the local standard of care) may be enrolled in the study if the participant fulfills all the following criteria: * Has a negative pregnancy test on the day of the first dose (Day 0). * Has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (Day 0). * Has agreed to continue adequate contraception through 3 months following the second dose (Day 28). * Is not currently breastfeeding. * Must be willing to provide verifiable identification (in accordance with the local regulations), has means to be contacted and to contact the investigator during the study. Exclusion Criteria: * History of confirmed infection of SARS CoV-2 prior to randomization; * Close contact with a confirmed COVID-19 within 14 days prior to randomization; * Prior administration of an investigational coronavirus vaccine or current/planned simultaneous participation in another interventional study to prevent or treat COVID-19; * Allergy to vaccines or vaccine/placebo ingredients, and serious adverse reactions to vaccines, such as urticaria, dyspnea, angioneuroedema; * Personal or first-grade relative (siblings) history of multisystem inflammatory disease in children (MIS-C); * Significant chronic illnesses that, in the opinion of the investigator, is at a stage where it might interfere with trial conduct or completion (may include, but are not limited to cardiovascular disease, liver or kidney disorders, respiratory illnesses) * Significant chronic central nervous system diseases or neuromuscular disorders, psychosis or severe cognitive behavioral disorder, in the opinion of the investigator, including epilepsy, autism spectrum disorder, intellectual disabilities (excluding Down Syndrome); * Acute central nervous system diseases such as encephalitis/myelitis, acute disseminating encephalomyelitis, and related disorders; * History of autoimmune and/or haematological diseases (including but not limited to systemic lupus erythematosus, thyroidectomy, autoimmune thyroid disease, any form of malignant tumor, asplenia, functional asplenia, or splenectomy resulting from any condition); well controlled type I diabetes mellitus is allowed; * History of bleeding disorders (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venipuncture; * Immunosuppressive therapy (systemic corticoid therapy, e.g. prednisone ≥2 mg/Kg/d or ≥20 mg/day for \>14 days), cytotoxic therapy (antineoplastic chemotherapy, radiation therapy), (excluding topical or aerosol corticosteroid therapy) in the past 6 months; * Receipt of blood products or immunoglobulins in the past 3 months; * Receipt of other investigational drugs in the past 30 days; * Receipt of attenuated live vaccines in the past 14 days; * Receipt of inactivated or subunit vaccines in the past 7 days; * Acute exacerbation or presentation of stable chronic diseases (including but not limited to asthma, migraine, gastrointestinal disorder, etc; * Acute febrile illness with oral temperature \>37.7°C or axillary temperature \>37.5°C on the day of vaccination (refer to section 7.1 Delay/Discontinuation of Study Vaccination); enrollment could be considered if the fever is absent for 72 hours; * Any confirmed or suspected human immunodeficiency virus (HIV) infection; * Children in care or under a court order; * According to the investigator's judgment, the subject has any other factors that might interfere with the results of the clinical trial or pose additional risk to the subject due to participation in the study.