Japanese encephalitis vaccine (Jevax) is an inactivated vaccine using the Nakayama viral strain. Until 2007, Jevax was the only Japanese encephalitis vaccine available in France but the duration of seroprotection after vaccination and exact timing of booster injections was unclear for travelers from non-endemic areas. The purpose of this report is to describe the results of a retrospective study in which neutralizing antibody levels were measured in 71 subjects previously vaccinated with Jevax. All subjects underwent testing at the Pasteur Institute Medical Center as part of preparation for humanitarian missions to endemic Japanese encephalitis areas in 2005-2006. A neutralizing antibody level greater than or equal to 20 was considered as protective. Findings showed that 49 of the 71 subjects (69%) still had protective antibody levels at a median of 4 years after the last Jevax immunization. In multivariate analysis, the only factor correlated with long-term seroprotection was the total number of vaccinations received. Based on these findings, it was concluded that long-term seroprotection after Jevax vaccination requires repeated booster injections even in subjects frequently exposed to the virus. No correlation was found between seroprotection and the interval between the booster injections.