The main study was a randomized, multi-nation, multi-centered, double-blinded, phase III study on subjects aged 2 months to <6 years old (<72 months). Two parts were conducted in Taiwan (Group A) and Vietnam (Group B). This extended study involved the subjects enrolled in the EV-BR1701 Group B1 who received any IP injections. For Group B, out of 2727 subjects enrolled from June 2022 to Jul 2023, 241 subjects of Group B1 would be assigned to this extension study. The extension study aimed to evaluate the immunogenicity persistence after EnVAX-A71 vaccination.

The subjects of Group B1 would be recalled for phlebotomy and immunogenicity analysis. The subjects would not receive further study EV71 vaccines in the extension study.

The principal investigators (PIs) and study coordinators (SCs) of the original medical center would contact the subject's guardians/legal representative by phone to return to the medical center for phlebotomy.

Informed consent would be provided to the guardians/legal representative of the subjects, and the eligibility of subjects would be verified on the day of the recall visit.

After the subject was evaluated by the PI and meets the phlebotomy requirements, the subject's guardian/legal representative would be asked to sign the informed consent form, and the new subject's number would be given.

About 3-4 mL of blood would be collected from each subject, and the serum would be collected and sent to the designated central laboratory for testing. The validated neutralizing antibody (NTAb) analysis method would be used to determine the neutralizing antibody titer.

The NTAb titer results would be directly sent to the CRO Data Manager Team, and statistical analysis for immunogenicity would be performed and reported after the main study was complete.

Inclusion Criteria:

1. Healthy children enrolled in group B1 of the EV-BR1701 study who received at least one dose of IP injection with the duration after the last vaccination from at least 2 years up to 2 years and 6 months.
2. The subject's guardians are able and willing to comply with study procedures and provide signed informed consent.

Exclusion Criteria:

1. Severe malnutrition or dysgenopathy at the screening visit.
2. Bleeding disorder diagnosed by a doctor or significant bruising or hemostatic difficulties with IM injections or phlebotomy.
3. Any condition in the investigator's opinion may interfere with evaluating study objectives.

The main study is a randomized, multi-nation, multi-centered, double-blinded, phase III study on subjects aged 2 months to <6 years old (<72 months). Two parts are conducted in Taiwan (Group A) and Vietnam (Group B). This extended study involves the subjects enrolled in the EV-BR1701 Group B1 who received any IP injections. For Group B, out of 2727 subjects enrolled from June 2022 to Jul 2023, 241 subjects of Group B1 will be assigned to this extension study. The extension study aims to evaluate the immunogenicity persistence after EnVAX-A71 vaccination.

The subjects of Group B1 will be recalled for phlebotomy and immunogenicity analysis. The subjects will not receive further study EV71 vaccines in the extension study.