A Study to Evaluate the Immunogenicity and Safety of EV71 Vaccine in Pediatric Subjects

Inclusion Criteria: 1. Healthy children aged from 3 to 6 years old (i.e. ≥ 3 years old and \< 7 years old) for Part A,and from 2 to 35 months old (i.e. ≥ 2 months old and \< 36 months old) for Part B at the time of first vaccination. 2. Subject's guardians are able and willing to comply with study procedures and provide the signed informed consent. 3. Subject is able and can comply with the requirements of the protocol. 4. Subject with body temperature ≤38°C. Exclusion Criteria: 1. Subject with previous known exposure to Enterovirus 71 (EV71). 2. Subject with a history of herpangina, hand-foot-mouth disease,and acute hemorrhagic conjunctivitis associated with enterovirus infection in the past 3 months. 3. Subject with gestation \< 37 weeks. 4. Subject with birth weight \<2.5 kg. 5. Subject with a history of hypersensitivity to vaccines, or a history of allergic disease or reactions likely to be exacerbated by any component of the vaccine. 6. Family history of seizures or progressive neurological disease. 7. Family history of congenital or hereditary immunodeficiency. 8. Severe malnutrition or dysgenopathy. 9. Major congenital defects or serious chronic illness, including perinatal brain damage. 10. Subject diagnosed of having autoimmune disease (e.g., celiac disease, type I diabetes, lupus (SLE), juvenile dermatomyositis, scleroderma, juvenile idiopathic arthritis (JIA), immune (or idiopathic) thrombocytopenia purpura). 11. Bleeding disorder diagnosed by a doctor or significant bruising or hemostatic difficulties with IM injections or blood draws. 12. Any acute infections 7 days prior to administrating the first vaccination. 13. Use of any investigational product (including drug, vaccine) within 30 days prior to vaccination or planned use during the study period. 14. Administration of any vaccines within 14 days prior to randomization. 15. Use of immunoglobulins or any blood products within 3 months prior to vaccination or planned use during the study period. 16. Chronic administration (defined as \> 14 days) of immunosuppressants or other immunomodulators or systemic corticosteroids within 6 months prior to vaccination or planned use during the study period. 17. Subjects who had ever received investigational EV-71 vaccine prior to randomization. 18. Under anti-tuberculosis prevention or therapy. 19. Any condition that in the opinion of the investigator may interfere with the evaluation of study objectives.