Elina O. ErraHelena Hervius AsklingLars RomboJukka RiuttaSirkka VeneSutee YoksanLars LindquistPakkanen, Sari H.Eili HuhtamoOlli VapalahtiAnu KanteleMahidol University. Faculty of Medicine. Haartman InstituteMahidol University. Institute of Molecular Bioscience2015-06-132017-04-252015-06-132017-04-252015-06-132012-06Clinical Infectious Diseases. Vol.55, No.6 (2012), 825-834https://repository.li.mahidol.ac.th/handle/20.500.14594/1822BACKGROUND: A significant part of the world population lives in areas with endemic Japanese encephalitis (JE). For travelers from nonendemic countries, Vero cell-derived vaccine (JE-VC; Ixiaro) has replaced traditional mouse brain-derived vaccines (JE-MB) associated with safety concerns. The 2 vaccines are derived from different viral strains: JE-VC from the SA14-14-2 strain and JE-MB from the Nakayama strain. No data exist regarding whether JE-VC can be used to boost immunity after a primary series of JE-MB; therefore, a primary series of JE-VC has been recommended to all travelers regardless of previous vaccination history. METHODS: One hundred twenty travelers were divided into 4 groups: Volunteers with no prior JE vaccination received primary immunization with (group 1) JE-MB or (group 2) JE-VC, and those primed with JE-MB received a single booster dose of (group 3) JE-MB or (group 4) JE-VC. Immune responses were tested before and 4-8 weeks after vaccination using plaque reduction neutralization test (PRNT) against both vaccine strains. RESULTS: In vaccine-naive travelers, the vaccination response rate for test strains Nakayama and SA14-14-2 was 100% and 87% after primary vaccination with JE-MB and 87% and 94% after JE-VC, respectively. Antibody levels depended on the target virus, with higher titers against homologous than heterologous PRNT(50) target strain (P < .001). In travelers primed with JE-MB, vaccination response rates were 91% and 91%, and 98% and 95% after a booster dose of JE-MB or JE-VC, respectively. Subgroup analysis revealed that a higher proportion of primed (98%/95%) than nonprimed (39%/42%) volunteers responded to a single dose of JE-VC (P < .001). CONCLUSIONS: A single dose of JE-VC effectively boosted immunity in JE-MB-primed travelers. Current recommendations should be reevaluated. CLINICAL TRIALS REGISTRATION: NCT01386827.engMahidol UniversitySingle DoseVero Cell–DerivedJapanese(JE)Vaccine(Ixiaro)Effectively Boosts ImmunityMouse Brain–DerivedVaccinesOpen Access articleArticleOxford University Press10.1093/cid/cis542.