The phase II study of inactivated vaccine (vero cell) against EV71 has completed last month in Jiangsu Province in China. The data from the phase II study suggested that the inactivated EV71 vaccine had a clinically acceptable safety and good immunogenicity for healthy Chinese children and infants. But the antibody titer against EV71 was found decreased significantly at month 8 compared to that at day 56. Considering the similar dynamic trend had also been found in the antibody against poliovirus induced by polio vaccines, and whose immunization schedule contains a booster dose administrated at 1 to 1.5 year after the first 3 doses of fundamental immunity. In order to find a better immunization schedule for children, the investigators decided to perform this booster immunity trial among these children who had received two doses of EV71 vaccines (around one year after the fundamental immunity). The investigators do the recruitment among these children who had participated in the previous phase II trial and received EV71 vaccines, and randomize them in a ratio of 2:1 to receive either a booster dose of EV71 vaccines or placebo.