Immunogenicity, Safety, and Immune Persistence of a Novel Inactivated Human Enterovirus 71 Vaccine: A Phase II, Randomized, Double-Blind, Placebo-Controlled Trial

Background. Vaccination is considered a top priority for the control of human enterovirus 71 (EV71) infection outbreaks. Methods. On the basis of phase I trial results, we conducted a double-blind, randomized, controlled trial to evaluate the optimal dose, immunogenicity, safety and immune persistence of the vaccine. A total of 480 healthy infants were randomly assigned to receive 2 injections of 100 U of vaccine, 200 U of vaccine, 400 U of vaccine, or placebo. Solicited adverse events (AEs) within 7 days and unsolicited AEs within 28 days after each vaccination were collected for safety evaluation. Blood samples were collected for neutralizing antibody assay. Results. EV71 vaccine was well tolerated, and no dose-related safety concerns were observed. Two doses of the vaccine yielded seropositivity frequencies of 92.3%, 95.9%, and 99.0% (with titers >= 1:8) in the 100 U, 200 U, and 400 U groups, respectively. Geometric mean titers measured by neutralizing antibody assay increased to 60.2 (95% confidence interval [CI], 41.9-86.4), 72.8 (95% CI, 50.8-104.3), and 252.1 (95% CI, 180.8-351.6) for the 100 U, 200 U, and 400 U groups, respectively. The dose-response relationship, with the 400 U dose showing higher immunogenicity than the 100 U and 200 U doses, remained until 13 months after the second vaccination, despite waning antibody levels. Conclusions. The 400 U dose was recommended as the optimal dose for the phase III trial because of its good safety prolile and higher immunogenicity.