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


BACKGROUND
NCT01386827.
Haartman Institute, Faculty of Medicine, University of Helsinki, Finland.
Elina O
EO
Erra
Helena Hervius
HH
Askling
Lars
L
Rombo
Jukka
J
Riutta
Sirkka
S
Vene
Sutee
S
Yoksan
Lars
L
Lindquist
Sari H
SH
Pakkanen
Eili
E
Huhtamo
Olli
O
Vapalahti
Anu
A
Kantele
D000293:Adolescent; D000328:Adult; D000368:Aged; D000818:Animals; D057134:Antibodies, Neutralizing / Q000097:blood; D000914:Antibodies, Viral / Q000097:blood; D002522:Chlorocebus aethiops; D004672:Encephalitis, Japanese / Q000276:immunology* / Q000517:prevention & control*; D005260:Female; D006801:Humans; D007117:Immunization, Secondary / Q000379:methods*; D022321:Japanese Encephalitis Vaccines / Q000008:administration & dosage / Q000276:immunology* / Q000302:isolation & purification; D008297:Male; D051379:Mice; D008875:Middle Aged; D009500:Neutralization Tests; D011446:Prospective Studies; D014195:Travel; D057236:Travel Medicine / Q000379:methods; D014709:Vero Cells; D010948:Viral Plaque Assay; D055815:Young Adult
D057134:Antibodies, Neutralizing; D000914:Antibodies, Viral; D022321:Japanese Encephalitis Vaccines