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

This study is an open,observational and follow-up clinical trial of EV71 vaccine based on the clinical trial of EV71 vaccine extended age group.The main purpose of this study is to evaluate the immunity persistence of EV71 vaccine developed by Sinovac Biotech Co., Ltd in subjects aged 6\~71 months after full immunization of two doses of vaccine.In this study, a total of 474 subjects from the EV71 vaccine extended age clinical trial who received two doses of experimental or control vaccine and were enrolled in the PPS set，including 274 subjects aged 6\~35 months (the maximum number of subjects aged 6-23 months is 146, and the maximum number of subjects aged 24-35 months is 128),100 subjects aged 36\~71 months and were enrolled in experimental group(EV71 vaccine developed by Sinovac Biotech Co., Ltd) and 100 subjects aged 36\~71 months and were enrolled in control group(EV71 vaccine developed by Institute of Medical Biology,Chinese Academy of Medical Sciences) will be enrolled .Blood samples will be collected from subjects aged 6-23 months at 36 months after full immunization and at the age of 72 months.Blood samples will be collected from subjects aged 24 to 35 months at 36 months after full immunization.Blood samples will be collected from subjects aged 36-71 months at 36 months after the full immunization.