evidence: In November 2012, we performed a randomized, double-blind, placebo-controlled trial in Donghai County, Jiangsu Province, China. This trial was conducted using a cohort of healthy children created in the early phase 2 clinical trial with EV71 vaccines, all of whom were aged 6–35 months. Participants in that cohort who had received at least 1 dose of EV71 vaccine in any formulations (160 U, 320 U, and 640 U with alum adjuvant or 640 U without adjuvant) composed the target population for recruitment in this booster immunization trial. Exclusion criteria were as follows: participants’ guardians did not provide informed consent, acute febrile disease was present on the day of recruitment, acute infection due to any cause was present during the week before recruitment, any inactivated vaccine was received during the week before recruitment, any live attenuated vaccine was received during the 2 weeks before recruitment, and any medical history of laboratory-confirmed EV71-associated disease, of allergy to any vaccination or drug, of allergy to any ingredient in the inactivated EV71 vaccine, and/or of seizures or other common contraindicating conditions was present. The eligibility of participants was assessed through medical history inquiry and physical examination by investigators.