The majority (93%) of vaccinees were found to have neutralizing antibodies against the vaccine strain two years after receiving

the JE-VC primary series. The longevity of the neutralizing antibodies against the vaccine strain has been investigated before [19], [20] and [21]. In these studies a somewhat shorter duration of seroprotection was shown for JE-VC; the current recommendation therefore is to give a booster dose at 12–24 months after the primary series [22]. The lowest seroprotection was observed in a study reporting a rate of 58% one year and 48% two years Birinapant after JE-VC primary immunization [20]. In another study, 69% of the subjects were found to be seroprotected at 15 months see more [21], while a third one reported a seroprotection rate of 83% at one year [19]. Such differences in long-term seroprotection have been associated with variations in the subjects’ TBE vaccination status [20]. In the present investigation, approximately half of the primary vaccinees had previously received a TBE or YF vaccine. The seroprotection rates proved high in subjects both with and without a history of other flaviviral vaccinations. Unfortunately, the limited number of participants did not allow specific analyses of the potential effect of TBE/YF vaccines on JE vaccination responses. However, these data suggest that a minimum booster interval

of two years can be considered at least for those

immunized with other flaviviral vaccines, and possibly also for the vaccine-naive. The emergence of heterologous JEV strains and genotypes has raised a question of the current JE vaccines’ 4-Aminobutyrate aminotransferase capacity to confer cross-protection against circulating strains of non-vaccine genotypes [10], [11] and [12]. In the present study, the majority of JE-VC-primed travelers showed protective levels of neutralizing antibodies against the six heterologous test strains representing genotypes GI–GIV at the two-year follow-up. The seroprotection rates against GI appeared lower than those against the other test strains (GII–GIV), yet these differences did not reach statistical significance. With respect to genotypes GII, GIII and GIV our data suggest an opportunity to extend the interval between primary series and first booster even longer than 24 months. This recommendation would, however, not be justified in light of the observation that only 73% of the vaccinees were seroprotected against GI after primary immunization with JE-VC; in fact, even a two-year interval could hence be criticized. The recent data proving that a single dose of JE-VC will suffice to elicit short-term protective response in JE-MB-primed travelers [5] and [6] has prompted some countries, such as Finland, to revise their national recommendations accordingly [23]. Until now, however, no data have been provided on protection duration.