• Deborah J Mills, Narayan Gyawali, Nirupama A Nammunige, Christine Mills, Gregor J Devine, Colleen L Lau, and Luis Furuya-Kanamori.
• Dr Deb The Travel Doctor, Travel Medicine Alliance, Brisbane, Australia.
• J Travel Med. 2025 Jan 21.

BackgroundJapanese encephalitis virus (JEV) is a leading cause of viral encephalitis in Asia, with high case-fatality rate and morbidity. Although the live recombinant Japanese encephalitis chimeric vaccine (Imojev®) offers strong initial immunity, data on long-term efficacy beyond five years remain limited.MethodsWe conducted a cross-sectional study on adults vaccinated with Imojev® at a specialist travel clinic in Brisbane, Australia. Participants were stratified based on the time since vaccination: 2-5 years and > 5 years. Neutralising antibody titres were measured using the plaque reduction neutralisation test (PRNT50), with titres ≥10 indicating seropositivity.ResultsOf the 103 participants, 47 were vaccinated 2-5 years prior and 56 were vaccinated ≥5 years prior to enrolment. All participants vaccinated within 5 years remain seropositive, while 52 of 56 (92.9%) vaccinated ≥5 years ago were seropositive. Four participants (7.1%) were seronegative post-vaccination, with time since vaccination ranging from 5 to 9 years. These seronegative individuals were vaccinated a median of 9.2 years ago, compared to 5.1 years for seropositive participants (p-value = 0.037). Aside from time since vaccination, no other factors (e.g. age, sex) were associated with seronegativity.ConclusionsImojev® provides durable immunity, with seropositivity exceeding 90% up to 10 years post-vaccination. However, waning immunity in a small proportion of individuals suggests that booster doses may be beneficial for high-risk travellers vaccinated over five years ago.© The Author(s) 2025. Published by Oxford University Press on behalf of International Societyof Travel Medicine.

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