This trial is designed as a single-center, randomized, double-blind study. A total of 720 infants aged 6 to 35 months are selected (with 240 subjects in each age group: 6-11 months, 12-23 months, and 24-35 months). Subjects from different age groups are randomly allocated to three batch groups in a 1:1:1 ratio, and receive two doses of the EV71 vaccine from the commercial production batch on days 0 and 30. After each vaccination dose, subjects will be observed for immediate reactions for 30 minutes and for solicited adverse events for the next 7 days, as well as for unsolicited adverse events over the 30 days following vaccination. All subjects will have blood samples collected on day 0 (pre-immunization) and day 60 for serum antibody testing. Additionally, 270 subjects will be selected, for additional blood sampling at various time points: 7 days, 14 days, and 30 days after the first dose, and 7 days and 14 days after the second dose, to assess the EV71 antibody levels at different timepoints following various doses of vaccination.

Inclusion Criteria:

• Infants and young children aged 6 to 35 months.
• Able to provide legal identification.
• The guardian of the subject has the ability to understand and agrees to sign the informed consent form.

Exclusion Criteria:

• Previous vaccination with the EV71 vaccine.
• History of hand, foot, and mouth disease.
• History of allergies, asthma, including allergies to vaccines or vaccine components, and severe adverse reactions to vaccines, such as urticaria, difficulty breathing, angioedema, or abdominal pain.
• Congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc.
• Autoimmune diseases or immune deficiencies/immunosuppression.
• Severe neurological disorders (epilepsy, convulsions, or seizures) or mental illness.
• History of thyroidectomy, asplenia, functional asplenia, or any condition leading to asplenia or splenectomy.
• Blood coagulation abnormalities diagnosed by a physician (such as clotting factor deficiencies, coagulopathies, platelet abnormalities) or evident bruising or bleeding disorders.
• Treatment with immunosuppressants, cytotoxic therapy, or inhaled corticosteroids within 6 months prior to enrollment (excluding corticosteroid nasal spray therapy for allergic rhinitis, or topical corticosteroid treatment for acute non-complicated dermatitis).
• Receipt of blood products within 3 months prior to vaccination with the trial vaccine.
• Receipt of other investigational vaccines within 30 days prior to vaccination with the trial vaccine.
• Receipt of live attenuated vaccines within 14 days prior to vaccination with the trial vaccine.
• Receipt of subunit or inactivated vaccines within 7 days prior to vaccination with the trial vaccine.
• Any acute illness or acute exacerbation of chronic illness within the past 7 days.
• Fever before receiving the investigational vaccine, with an axillary temperature > 37.0°C.
• Any other factors that, in the investigator's judgment, make the subject unsuitable for participation in the clinical trial.