The purpose of this study is to evaluate the 5-year Immune Persistence of Inactivated Enterovirus Type 71 (EV71) Vaccine manufactured by Sinovac (Beijing) Biotech Co., Ltd.

The phase III, efficacy trial of inactivated vaccine (vero cell) against EV71 has completed on March 2013 in China,and a follow up study for phase III clinical trial has completed on March 2014.

On the basis of phase III, this study is the Phase IV, open-labelled research, in order to evaluate the 5-year immune persistence of 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

The studied subjects are infants aged 6-35 months old infants when enrolled into the phase III clinical trial, and they are children aged 6-8 years old five years after the EV71 vaccine /placebo injection.