An Immunity Persistence Study of Enterovirus 71 Inactivated Vaccine (Vero Cell)

Inclusion Criteria: * Subjects in the EV71 vaccine extended age clinical trial who received two doses of experimental or control vaccine and were enrolled in the PPS set; * Provide legal identification; * Participants or guardians of the participants should be capable of understanding the written consent form and voluntarily participate in the clinical trial. Exclusion Criteria: * Vaccination history of vaccines containing EV71 antigen components other than the EV71 vaccine expanded age group clinical trial; * History of hand, foot and mouth disease caused EV71; * According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial.