Recently, an inactivated vaccine (vero cell) against EV71 has been investigated in Phase 1 and Phase 2 clinical trials. Data from these trials showed that the EV71 vaccine has good safety profile and was immunogenic. 320 U alum-adjuvant vaccine has been chosen as the candidate vaccine for the phase 3 clinical trial.

This clinical trial is a supplementary phase 2 trial, which is designed to study the gene expression patterns induced by EV71 vaccine in Chinese healthy children aged from 2 to 5 years old use a systems biology approach combined with microarray analysis，RT-PCR and neutralizing antibody testing for PBMC and serum collected form the studied children population, to predict immunogenicity, and explore mechanistic insights about the EV71 vaccine.

Inclusion Criteria:

• Healthy subjects aged from 2 to 5 years old as established by medical history and clinical examination
• The pre-vaccination neutralizing antibody against EV71 <1:8 which is determined by ELISA
• The subjects' guardians are able to understand and sign the informed consent
• Had never received the vaccine against EV71
• Subjects who can and will comply with the requirements of the protocol
• Subjects with temperature <37.1°C on axillary setting

Exclusion Criteria:

• Subject who has a medical history of HFMD
• <= 37 weeks gestation
• Subjects with a birth weight <2.5 kg
• Subject that has a medical history of any of the following: allergic history, or allergic to any ingredient of vaccine
• Family history of seizures or progressive neurological disease
• Family history of congenital or hereditary immunodeficiency
• Severe malnutrition or dysgenopathy
• Major congenital defects or serious chronic illness, including perinatal brain damage
• Autoimmune disease
• Bleeding disorder diagnosed by a doctor or significant bruising or bleeding difficulties with IM injections or blood draws
• Any acute infections in last 7 days
• Any prior administration of immunodepressant or corticosteroids in last 6month
• Any prior administration of blood products in last 3 month
• Any prior administration of other research medicines in last 1month
• Any prior administration of attenuated live vaccine in last 28 days
• Any prior administration of inactivated vaccines in last 14 days, such as pneumococcal vaccine
• Under the anti - TB prevention or therapy
• Any condition that in the opinion of the investigator, may interfere with the evaluation of study objectives