A single dose of vero cell-derived Japanese encephalitis (JE) vaccine (ixiaro) effectively boosts immunity in travelers primed with mouse brain-derived JE vaccines

Abstract

Background. 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 PRNT50 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. © 2012 The Author.