Inactivated Bivalent Enterovirus Vaccine (Vero Cell) Phase I/II Clinical Trial

The Phase I clinical trial is divided into two parts. The first part uses an open-label design, while the second part uses a randomized, double-blind, and controlled design. The goal of this clinical trial is to evaluate the tolerability and safety of a bivalent (EV71/CA16, Enterovirus 71/Coxsackievirus A16) inactivated enterovirus vaccine (Vero cell) developed by Sinovac. The vaccine will be administered to healthy adults, children aged 6 to 12 years, and children aged 6 to 71 months. The Phase II clinical trial will also use a randomized, double-blind, and controlled design to evaluate the safety and immunogenicity of the same bivalent inactivated enterovirus vaccine (Vero cell) developed by Sinovac. This trial will involve healthy children aged 6 to 71 months.

Inclusion Criteria: * Healthy participants aged 18-59 years, 6-12 years, and 6-71 months. * Participants and/or their guardians can understand and voluntarily sign the informed consent form (participants aged 8-12 years require dual signatures from both participants and their guardians). * Willing and able to comply with all visit schedules, sample collection, vaccine administration, and other trial procedures. * Provide legal identification of the participants and/or their guardians. Exclusion Criteria: * History of hand, foot, and mouth disease or EV71 vaccination. * Known severe allergy to the vaccine or vaccine components, such as urticaria, respiratory distress, and angioedema. * Congenital abnormalities or developmental disorders, genetic defects (such as Down syndrome, thalassemia, or G6PD deficiency), severe malnutrition, etc. * Based on medical history and physical examination, the presence of severe chronic diseases, severe cardiovascular diseases, uncontrolled hypertension or diabetes, liver or kidney diseases, etc. * Based on medical history and physical examination, the presence of autoimmune diseases, immunodeficiency diseases, and/or blood diseases (including but not limited to systemic lupus erythematosus, ankylosing spondylitis, thyroidectomy, autoimmune thyroid disease, any form of malignancy, asplenia, functional asplenia). * Based on medical history and physical examination, abnormal coagulation function (such as coagulation factor deficiency, coagulation disorders, platelet abnormalities) or obvious bruising or bleeding disorders. * Based on medical history and physical examination, the presence of severe neurological diseases (epilepsy, convulsions, or seizures), mental illness, or corresponding family history. * Long-term use (more than 14 days) of corticosteroids or other immunosuppressive agents (excluding corticosteroid sprays for allergic rhinitis and surface corticosteroid treatment for acute non-complicated dermatitis) or cytotoxic therapy or planning to receive such treatment during the trial. * History of long-term alcohol abuse or drug abuse. * Females who are breastfeeding, pregnant, or planning to become pregnant within 90 days, or females with positive pregnancy test results. Note: male participants whose spouse plans to become pregnant within 90 days are also included. * Received immunoglobulin and/or other blood products in the past 3 months or planning to receive such treatment during the trial. * Received other investigational drugs or vaccines within the past 30 days or planning to receive such drugs or vaccines during the trial. * Vaccinated with attenuated live vaccines within the past 14 days or subunit or inactivated vaccines within the past 7 days. * Suspected or confirmed fever within 72 hours before vaccination or axillary temperature \>37.0℃ on the day of vaccination. * Clinical laboratory testing shows laboratory abnormalities beyond the reference range and with clinical significance (applicable only to Phase I clinical trials for adults/6-12-year-old children/24-71-month-old children): A. Complete blood count: white blood cell count, hemoglobin, platelet count.B. Blood biochemistry: alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin (TBIL), creatinine (CR), fasting blood glucose (Glu).C. Urine routine: urine protein (PRO). * Any acute illness (with or without fever) or exacerbation of chronic illness within the past 7 days. * According to the investigator's judgment, there are any other factors that make the participant unsuitable for participation in the clinical trial.