Antibody Persistence up to 3 Years After Primary Immunization With Inactivated Japanese Encephalitis Vaccine IXIARO in Philippine Children and Effect of a Booster Dose

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Background:An inactivated Vero cell culture derived Japanese encephalitis virus vaccine (IXIARO) requires a booster dose 1 year after primary schedule for long-term antibody persistence in adults. The aim of this study is to evaluate immunogenicity and safety of a booster dose in children 2 months to <18 years of age.Methods:This is a randomized, controlled open-label study in the Philippines. Three hundred children vaccinated with IXIARO in a previous trial were randomized 1:1 to receive either no booster or a booster 12 months after initiation of the primary series. Neutralizing antibody titers were assessed before and after the booster and up to 3 years after primary series. Safety endpoints included the rate of subjects with solicited adverse events (AEs), unsolicited AEs and serious AEs within 1 month after the booster.Results:Geometric mean titer declined by 1 year after the primary series, but titers remained above the established protective threshold in 85%–100% of children depending on age group. The booster led to a pronounced increase in geometric mean titer and 100% seroprotection rate in all age groups. The booster was well tolerated, with AE rates lower compared with the primary series. Most AEs were mild.Conclusions:A booster dose of IXIARO administered 12 months after the primary immunization was well tolerated and highly immunogenic.

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