Clinical Trial for Batch Consistency of EV71 Vaccine

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.