A Booster Dose of an Inactivated Enterovirus 71 Vaccine in Chinese Young Children: A Randomized, Double-Blind, Placebo-Controlled Clinical Trial

Background. A significant waning of enterovirus 71 (EV71) antibody titer after priming immunization with an inactivated EV71 vaccine implied the potential need for a booster dose. Methods. In this randomized, double-blind, placebo-controlled clinical trial, we recruited participants who had received at least 1 dose of priming EV71 vaccine in an early phase 2 clinical trial that was conducted in healthy infants and children aged 6-35 months. All participants were grouped according to the priming EV71 vaccine formulations (160 U, 320 U, and 640 U with adjuvant and 640 U without adjuvant) and then randomly assigned (ratio, 2:1) to receive a booster dose of vaccine or placebo within each formulation group. The primary end point was the geometric mean titer 28 days after the booster dose. Results. A total of 773 participants were enrolled. Significantly greater immunological responses were induced by the booster shot of all 4 formulations of EV71 vaccine, compared with that induced by placebo (P