High Seroprotection Rates among Adults 10 Years after Inactivated Vero Cell-Derived Japanese Encephalitis Vaccine, IXIARO

Japanese encephalitis (JE), a mosquito-borne, vaccine-preventable disease, is a risk in parts of Asia and Oceania. A previous study showed that after a primary 2-dose series and booster dose of the Vero cell-derived JE vaccine (JE-VC; IXIARO), high seroprotection rates in adults persisted through 6 years. We used serum samples from U.S. military personnel aged 29–55 years to investigate seroprotection at 10 years after the booster dose (based on a 50% plaque reduction neutralization test titer of ≥10). Overall, 97% (95% confidence interval [CI]: 94–98%) of 300 individuals were seroprotected at approximately 10 years with a geometric mean titer of 165 (95% CI: 140–195). Neutralizing antibody titers were not significantly associated with age, intervals between primary series or booster doses, or previous yellow fever vaccination. Young and middle-aged adults who have completed a JE-VC primary series and booster maintain high seroprotection rates for at least 10 years.