Hand, Foot, and Mouth Disease（HFMD)

This multicenter, randomized, double-blind, placebo-controlled phase IIIa clinical trial aims to evaluate the protective efficacy , immunogenicity and safety of the Tetravalent Inactivated Enterovirus Vaccine (Vero Cell) in Children aged 6 to 71 months. Participants will be randomly assigned in a 1:1 ratio to the trial group and the placebo group, receiving two doses of experimental vaccine or placebo , with a one-month interval between the two doses.

This multicenter, randomized, double-blind, controlled Phase III clinical trial aims to evaluate the efficacy, safety, and immunogenicity of the bivalent enterovirus-inactivated vaccine (Vero cell) in healthy children aged 6 to 71 months. The main questions it aims to answer are: * The primary vaccine efficacy of the investigational vaccine against Hand, Foot, and Mouth Disease（HFMD) caused by CA16 infection compared to the control vaccine. * The neutralizing antibody levels against EV71 in the trial group are non-inferior to those in the control group after two doses of vaccination. Researchers will compare the bivalent enterovirus-inactivated vaccine (Vero cell) to the EV71-inactivated vaccine (Vero cell) to prevent HFMD and Herpangina(HA). Participants will be randomly assigned to the trial group and the control group in a 1:1 ratio to receive two doses of the investigational vaccine or the control EV71 vaccine, with a one-month interval between doses.