[Neutralizing antibody titer after recommended early vaccination against Japanese encephalitis]

Objective　Following the case of a 10-month-old with Japanese encephalitis in Chiba in 2015, the Chiba Pediatric Association and Chiba Medical Association recommended that children receive the Japanese encephalitis vaccine at six months of age. In this study, we investigated early vaccinations and standard vaccinations of children in a patient outbreak area. Additionally, we examined changes in the timing of vaccination initiation and the acquisition and maintenance of protective immunity before and after the early vaccination recommendation.Methods　With the cooperation of the medical institution designated for Type II Infectious Diseases, which treated affected children in 2015, the history and timing of vaccinations for children who visited the hospital between October 2018 and March 2020 were investigated, and serum samples were collected. The data obtained in this study and the Ministry of Health, Labour and Welfare's Community Health Service Report were used to investigate the timing of vaccinations. Neutralizing antibody titers in serum were quantified with the focus measurement method and examined with respect to the number of vaccinations, doses, and number of days since vaccination.Results　A total of 154 samples were obtained, with 89 from the vaccinated group and 65 from the unvaccinated group. There was no difference in the percentage of children based on the age of first vaccination between Japan, Chiba Prefecture, and the study subjects until FY2015, but from FY2016 onward, the percentage of children vaccinated early was higher among the study subjects, as well as for children from Chiba Prefecture and Japan, in that order. The antibody possession rate by vaccination frequency was 9.2% for the unimmunized, 87.5% for those vaccinated once, 95.1% for those vaccinated twice, and 100% for those vaccinated three times. In the group vaccinated twice, antibody titers of children who received half the vaccine dose were significantly lower than those of children who received the normal dose.Conclusion　In the districts targeted in this study, significantly more children under three years of age were vaccinated before the early vaccination recommendation, suggesting the effectiveness of that recommendation. In addition, the high rate of antibody positivity among unvaccinated children suggests that it is important to promote early vaccination in areas at high risk for Japanese encephalitis. In this study, there was no evidence of attenuation of antibody titers over time after early immunization, but the possibility remains that the children who completed early immunization received a booster due to pathogen exposure and maintained antibody titers, given that Japanese encephalitis was endemic in the target area.