This study is designed to evaluate the long-term antibody titers of EV71 vaccine 4 and 5 years after first dose vaccination for subjects at the age of 2 to < 6 years , and 3 to 5 years after first dose vaccination for subjects at the age of 2 months to < 2 years

This is an extension study of protocol CT-EV-21, to evaluate the long-term immunogenicity of EV71vaccine.

The first study visit will be approximately 3 or 4 years after the administration of first vaccination of EV71 vaccine or placebo in CT-EV-21 study; 4 years after the first vaccination for subjects 2 to < 6 years of age (2b), and 3 years after first dose for subjects 2 months to < 2 years of age (2c and 2d).

Subjects of 2b will remain in the study for approximately 12 months and will have 2 clinic visits; subjects of 2c and 2d will remain in the study for approximately 24 months and will have 3 clinic visits. Immunogenicity response against EV71 virus antigen at each visit will be assessed.

Inclusion Criteria:

• Subjects who have completed participation in study CT-EV-21 part 2b, 2c, and 2d, and have received protocol specified doses of EV71 vaccine or Placebo (total of 2 doses in part 2b, and 3 doses for part 2c and 2d).
• The subjects' guardians are able to understand and sign the informed consent form.

Exclusion Criteria:

• Subjects and/or guardians who refuse to comply with the study procedures

Subjects who have completed participation in study CT-EV-21