Protective Efficacy , Immunogenicity and Safety of the Tetravalent Inactivated Enterovirus Vaccine (Vero Cell).

Inclusion Criteria: 1. Healthy children aged 6 to 71 months. 2. Guardians who can understand and voluntarily sign the informed consent form 3. Willing and able to comply with all visit schedules, sample collection, vaccinations, and other trial procedures。 4. Provide legal identification for the participant and their guardian Exclusion Criteria: 1. A known history of HFMD/HA 2. Uncontrolled chronic diseases or a history of severe illnesses, including but not limited to cardiovascular diseases, blood disorders, liver or kidney diseases, digestive system diseases, respiratory system diseases, malignant tumors, or a history of major functional organ transplantation. 3. Autoimmune diseases, immunodeficiency diseases (including but not limited to systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid diseases, asplenia, functional asplenia, and HIV infection). 4. Abnormal coagulation function (such as coagulation factor deficiencies,and platelet abnormalities). 5. Suffering from/having a history of severe neurological diseases (epilepsy, convulsions, or seizures \[excluding a history of febrile seizures\]) ,psychiatric disorders, or a family history of psychiatric disorders 6. Various acute diseases or exacerbations of chronic diseases within the last 3 days. 7. Having been vaccinated with a vaccine containing any of the components EV71, CA16, CA10, CA6. 8. Having received ≥14 days of immunosuppressive or other immunomodulatory treatment (prednisone ≥2mg/kg/day, or its equivalent; local or inhaled corticosteroids excluded) within the past 6 months, or cytotoxic treatment, or planning to receive such treatment during the trial. 9. Having received immunoglobulin or other blood products(excluding hepatitis B immunoglobulin) within the past 6 months, or planning to receive such treatment during the trial. 10. Having received other investigational drugs or vaccines within the past 30 days, or planning to receive such drugs or vaccines during the trial. 11. Having received live attenuated vaccines or nucleic acid vaccines within the past 14 days, or subunit or inactivated vaccines within the past 7 days. 12. Known allergy to any component of the investigational vaccine (inactivated EV71 virus, inactivated CA16 virus, , inactivated CA10 virus,, inactivated CA6 virus,aluminum hydroxide, sodium chloride, disodium hydrogen phosphate, sodium dihydrogen phosphate, injectable water). 13. On the day of planned vaccination with the investigational vaccine, having an axillary temperature ≥37.3°C before vaccination, or other vital sign measurements outside the normal range,or the physical examination is not qualified. 14. According to the investigator's judgment, participants have any other factors that make them unsuitable for participation in the clinical trial.