Elina O. ErraHelena Hervius AsklingSutee YoksanLars RomboJukka RiuttaSirkka VeneLars LindquistOlli VapalahtiAnu KanteleUniversity of Helsinki Haartman InstituteHelsinki University HospitalKarolinska InstitutetMahidol UniversitySormland County CouncilMedical Centre AavaSwedish Institute for Communicable Disease ControlHelsingin Yliopisto2018-10-192018-10-192013-12-17Vaccine. Vol.32, No.1 (2013), 119-123187325180264410X2-s2.0-84891935899https://repository.li.mahidol.ac.th/handle/20.500.14594/31138Background: The inactivated Vero cell-derived vaccine (JE-VC, IXIARO) has replaced the traditional mouse brain-derived preparations (JE-MB) in travelers' vaccinations against Japanese encephalitis. We showed recently that a single JE-VC dose efficiently boosts immunity in JE-MB-primed vaccinees, and that JE-VC elicits cross-protective immunity against non-vaccine genotypes, including the emerging genotype I. While these studies only provided short-term data, the present investigation evaluates the longevity of seroprotection in the same volunteers. Methods: The study comprised 48 travelers who had received (1) JE-VC primary series, (2) JE-MB primary series followed by a single JE-VC booster dose, or (3) JE-MB primary series and a single JE-MB booster dose. Serum samples were collected two years after the last vaccine dose, and evaluated with the plaque-reduction neutralization test against seven Japanese encephalitis virus strains representing genotypes I-IV. PRNT50titers≥10 were considered protective. Results: Two years after the primary series with JE-VC, 87-93% of the vaccinees proved to be cross-protected against test strains representing genotypes II-IV and 73% against those of genotype I. After a single homologous or heterologous booster dose to JE-MB-primed subjects, the two-year seroprotection rates against genotype I-IV strains were 89-100%. Conclusions: After JE-VC primary series, seroprotection appeared to wane first against genotype I. The first booster should not be delayed beyond two years. In JE-MB-primed subjects, a single JE-VC booster provided cross-protective immunity against genotype I-IV strains in almost all vaccinees, suggesting an interval of two years or even longer for the second booster. These data further support the use of a single JE-VC dose for boosting JE-MB immunity. © 2013 The Authors.Mahidol UniversityBiochemistry, Genetics and Molecular BiologyImmunology and MicrobiologyMedicineVeterinaryArticleSCOPUS10.1016/j.vaccine.2013.10.055