The objectives of this study are to evaluate the immune response and safety profiles of two injections of EV71 vaccine administrated with or without adjuvant AlPO4 at 0.5-μg, 1-μg, 2-μg and 5-μg dose (if required) in children aged 3 to 6 years old and 0.25-μg (if required), 0.5-μg, 1-μg and 2-μg (if required) in 6 to 35 months old infants/toddlers.

Inclusion Criteria:

1. Healthy children aged from 3 to 6 years old (i.e. ≥ 3 years old and < 7 years old) for Part A; from 6 to 35 months old (i.e. ≥ 6 months old and < 36 months old) for Part B at the time of first vaccination.
2. Subject's guardians were able and willing to comply with study procedures and give written informed consent.
3. Subject was able and could 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, 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 bleeding difficulties with IM injections or blood draws.
12. Any acute infections 7 days prior to administrate 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 attenuated live vaccine within 7 days prior to vaccination.
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.
17. Under anti-tuberculosis prevention or therapy.
18. Any condition that in the opinion of the investigator may interfere with the evaluation of study objectives.