An Extension Study of EV-BR1701 Part A1

I. Study Design: EV-BR1701 was a randomized, multi-nation, multi-centered, double-blinded, phase III study to evaluate the efficacy and safety of an EV71 vaccine, EnVAX-A71, in healthy infants and children in Taiwan (Part A) and Vietnam (Part B). In both Parts, a sub-study for immunogenicity assessment for up to one year was performed. This extension study of EV-BR1701 aims to evaluate the long-term immunogenicity persistence more than 5 years after EnVAX-A71 vaccination in subjects originally participating in the immunogenicity sub-study in Taiwan. The evaluable subjects of EV-BR1701 Part A1(immunogenicity sub-study), who were aged 2 months to \< 6 years when entering the main study and completed two injections of either EnVAX-A71 or placebo with evaluable data for the primary immunogenicity endpoint, will be recalled to enter this extension study for phlebotomy and immunogenicity analysis. These subjects will not receive further vaccination according to this extension study protocol. The principal investigators (PIs) and study coordinators (SCs) of the original medical center will contact the subject's guardians/legal representative by phone to return to the medical center for phlebotomy. Informed consent will be provided to the guardian/legal representative of the subjects, and the eligibility of subjects will be verified on the day of the recall visit. After the subject is evaluated by the PI and meets the phlebotomy requirements, the subject's guardian/legal representative will be asked to sign the informed consent form, and the subject will be given the same identifier as what he/she was originally assigned in the main study for continuity of his/her immunogenicity data. About 3-4 mL of blood will be collected from each subject, and the serum will be collected and sent to the designated central laboratory for testing. The validated neutralizing antibody (NTAb) analysis method will be used to determine the EV71-specific neutralizing antibody titer. The NTAb titer results will be directly sent to the CRO Data Management Team, and statistical analysis for immunogenicity will be performed and reported.