Five-year Immune Persistence Study of Inactivated Enterovirus Type 71 (EV71) Vaccine

Inclusion Criteria: In the prior phase III clinical trial, 10077 subjects in three counties (Ganyu, Sheyang and Taixing ) were enrolled ( Experimental group: 5044; Control group: 5043 ), with 1293 of them in immunogenicity subgroup (Experimental group: 648; Control group: 645). In Sheyang county, 3351 subjects were enrolled (Experimental group: 1676 ; Control group :1675) , with 435 of them in immunogenicity subgroup (Experimental group: 418 ; Control group :418). The subjects in Sheyang immunogenicity subgroup with the following conditions were included in this study: * Guardian(s) of the volunteer should be capable of understanding the written consent form, and such form should be signed before the infant being included into this study * Received at least one injection of EV71 vaccine or the placebo in the phase III clinical trial * Finished the blood sampling 64 months after the vaccination Exclusion Criteria: * Received extra EV71 vaccination after the phase III clinical trial * Refused to join the study