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 profile and higher immunogenicity.