A Study to Evaluate the Efficacy, Safety, and Immunogenicity of an EV71 Vaccine

Inclusion Criteria: 1. Healthy children and infants aged from 2 months to less than 6 years as established by medical history and clinical examination. 2. The subjects' parents/guardians are able to understand and sign the ICF. Exclusion Criteria: 1. Age \< 2 months or ≥ 6 years. 2. For subjects \< 1 year, gestational age \< 34 weeks or birth weight \< 2200 grams. 3. Has poor venous access (for subjects in sub-study only). 4. Currently has a fever defined as ear/rectal temperature ≥ 38°C or axillary temperature ≥ 37.5 °C within 2 days before the time of planned vaccination. 5. Has had previous known exposure to EV71 or has received EV71 vaccine. 6. Has a history of herpangina or HFMD associated with enterovirus infection in the past 30 days before the date of the planned study drug vaccination. 7. Has been diagnosed with a significant neurological, pulmonary, cardiovascular, hematological, hepatic, or renal disorder. 8. Has a history of hypersensitivity to vaccines, or a history of allergic disease or reactions likely to be exacerbated by any component of the vaccine. 9. Has used any investigational/non-registered product (including drug, vaccine, or invasive medical device) within 28 days before vaccination or plan to use during the study period. 10. Has confirmed or suspected autoimmune disorder, or immunodeficiency. 11. Administration of any licensed live attenuated vaccine within 14 days before or after each study vaccination (with the exceptions of live attenuated Rotavirus vaccine via oral route). 12. Has used immunoglobulins or any blood products within 11 months before vaccination or plan to use during the whole study period. 13. Has had immunosuppressants, cytotoxic drugs, corticosteroids (including prednisolone ≥ 0.5 mg/kg/day or equivalent), or immunomodulators taken for \> 14 days within 6 months before first vaccination or plan to use before the last scheduled study visit. (Inhaled and topical steroids are allowed). 14. Has any medical or psychiatric condition, that is a contraindication to protocol participation based on the judgment of the Investigator.