Long-term immunity and immune response to a booster dose following vaccination with the inactivated Japanese encephalitis vaccine IXIARO®, IC51

IC51 (IXIARO®, JESPECT®) is a recently approved prophylactic Japanese encephalitis virus vaccine with a two-vaccine primary immunization regimen. In this phase 3 trial, after primary immunization with a Day 0/28 dose schedule, seroprotection rates were 83%, 58% and 48% at Month 6, Month 12 and Month 24, respectively. A booster dose at Month 11 and/or Month 23 in subjects with neutralizing antibody titers below the limit of detection (defined as a serum dilution giving a 50% reduction of plaque counts in a plaque reduction neutralization test [PRNT50] < 1:10) led to 100% seroconversion. After a single-dose immunization (incomplete primary immunization), only 9% of subjects were seroprotected at Month 6; however, a booster dose at Month 11 led to seroconversion in 99% of subjects. Hence, subjects with incomplete primary immunization can complete their schedule within at least 11 months.