Safety and Immunogenicity of a Single Administration of Live-attenuated Japanese Encephalitis Vaccine in Previously Primed 2- to 5-year-olds and Naive 12- to 24-month-olds: Multicenter Randomized Controlled Trial

Background: Safe and effective Japanese encephalitis (JE) vaccines are needed to protect populations living in or visiting endemic areas. A live-attenuated JE-chimeric virus vaccine (JE-CV) has been developed with a single-dose regimen. Methods: In an open-label, crossover study, 100 children aged 2 to 5 years with a history of 2-dose primary vaccination with mouse-brain derived inactivated JE vaccine according to the Thai Expanded Program for Immunization schedule, and 200 JE vaccination-naive 12- to 24-month-old toddlers were randomized 1:1 to receive JE-CV, containing ≥4 log10 plaque forming units, 1 month before or after hepatitis A control vaccine. Neutralizing antibody titers were assessed using PRNT50 (titers expressed in inverse of dilution) before and 28 days after JE-CV, and at months 7 and 12. Results: All 2- to 5-year-olds and 96% of 12- to 24-month-olds were seroprotected (titer ≥10) 28 days after JE-CV administration, and geometric mean titers (GMT) (95% confidence interval) in these age groups were 2634 (1928–3600) and 281 (219–362), respectively. One year later, seroprotection rates in the 2 age groups were 97% and 84% and GMTs were 454 and 62.3, respectively. Vaccine-induced antibodies neutralized a panel of wild-type JE isolates. There were no vaccine-related serious adverse events. Reactogenicity of JE-CV was comparable with that of the inactivated hepatitis A vaccine. Conclusions: A single administration of JE-CV has a good safety profile and elicits a protective immune response in both JE-naive toddlers and JE-primed young children.