Inclusion Criteria:  Aged 6-17 years； The subject and/or guardian can understand and voluntarily sign the informed consent form (double signature is required for 8-17 years old)； Reproductive women have a negative pregnancy test before each vaccination； The subjects are able and willing to comply with the requirements of the clinical trial protocol, and can complete the study follow-up for approximately 13.5 months.  Exclusion Criteria:  History of SARS-CoV-2 infection. The COVID-19 antibody (IgG and IgM) screening was positive. History of asthma, history of allergy to vaccines or vaccine components, or severe adverse reactions to vaccines, such as urticaria, dyspnea, and angioedema. Congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc. Autoimmune diseases or immunodeficiency/immunosuppression. Severe chronic diseases that cannot be controlled by drugs, severe cardiovascular diseases, diabetes, liver and kidney diseases, malignant tumors, etc. Severe neurological disease (epilepsy, convulsions or convulsions) or mental illness. Any condition caused by thyroid disease or history of thyroidectomy, hypersplenism, or splenectomy. Diagnosis of abnormal blood clotting function (for example, lack of clotting factors, coagulopathy, abnormal platelets) or obvious bruising or blood clotting. In the past 6 months, received immunosuppressive therapy, cytotoxic therapy, inhaled corticosteroids (excluding corticosteroid spray therapy for allergic rhinitis, and superficial corticosteroid therapy for acute uncomplicated dermatitis). Laboratory test results with clinically significant abnormalities (severity level 2 and above) (applicable to phase I clinical trials only). Received blood products in the last 3 months. Receive other study drugs or vaccines within the past 30 days. Vaccination against live attenuated vaccines in the past 30 days. Inactivated vaccine or subunit vaccine in the past 14 days. Acute disease or acute exacerbation of chronic disease in the last 7 days. Axillary temperature> 37.0°C. According to the judgment of the investigator, the subject has other factors that are not suitable for participating in the clinical trial.  Subsequent dose exclusion criteria:  In this trial, the second/third dose of vaccination may be stopped in some cases. This includes allergic reactions, severe hypersensitivity reactions, or grade 3 or higher adverse reactions that cannot be tolerated after previous vaccination/placebo. If these reactions occur, the subject should not continue to receive the second/third vaccination.