Long-term immunogenicity of a single-dose live recombinant chimeric Japanese encephalitis vaccine in adults

Background: Japanese encephalitis virus 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 (R)) offers strong initial immunity, data on long-term efficacy beyond 5 years remain limited. Methods: We conducted a cross-sectional study on adults vaccinated with Imojev (R) at a specialist travel clinic in Brisbane, Australia. Participants were stratified based on the time since vaccination: 2-5 years and >5 years. Neutralizing antibody titres were measured using the plaque reduction neutralization test (PRNT50), with titres >= 10 indicating seropositivity. Results: Of 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, whilst 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. Conclusions: Imojev (R) 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 5 years ago.