Study on the Persistence of Immunity Following EV71 Vaccination

Inclusion Criteria: * Included in the immunogenicity group of initial studyand randomly into either vaccine (fulfilled the two doses vaccination following the protocol) or placebo group. * Volunteers and their legal guardians or appointed representatives must provide valid legal identification documents. * Legal guardians or appointed representatives of volunteers must have the capacity to understand the informed consent document and the research process, voluntarily participate, sign the informed consent form, and be able to comply with the study's requirements. Exclusion Criteria: * Had history of extra EV71 vaccination. * Previous history of hand-foot-mouth disease. * Participating in other research. * Subjects with a hereditary bleeding tendency or coagulopathy, or a history of bleeding disorders. * Investigators' Discretion: The final exclusion criterion is the discretion of investigators to determine whether a subject is suitable for participation in the study.