This study is designed to evaluate the long-term antibody titers of Envacgen® approximately 6 years after the first vaccination dose in participants who were aged 2 months to <6 years at the time of enrollment in the CT-EV-31s study.

This is an extension study of protocol CT-EV-31s, to evaluate the long-term immunogenicity of EV71 Enterovirus Vaccine (Envacgen®).

The study will consist of a single visit approximately 6 years after the first vaccination of the EV71 vaccine or Placebo in participants aged 2 months to < 6 years when participating in the CT-EV-31s study. Immunogenicity response against EV71 virus antigen at the visit will be assessed.

Inclusion Criteria:

• Participants who have completed participation in study CT-EV-31s and have received protocol specified doses of EV71 vaccine or Placebo.
• The participant and/or participant's parent/guardian is able to understand and sign the informed consent form.

Exclusion Criteria:

• The participant and/or participant's parent/guardian refuse to comply with the study procedures.
• After completing the CT-EV-31s trial, participants who received any EV71 vaccine before participating in this study.

Subjects who have completed participation in study CT-EV-31s.