• Luis Furuya-Kanamori, Narayan Gyawali, Deborah J Mills, Christine Mills, Leon E Hugo, Gregor J Devine, and Colleen L Lau.
• UQ Centre for Clinical Research, Faculty of Medicine, The University of Queensland, Herston, Australia.
• J Travel Med. 2023 Apr 5; 30 (2).

BackgroundJapanese encephalitis (JE) is endemic in Asia and the western Pacific. Vaccination is recommended for travellers to endemic regions, but the high cost of the vaccine is a major barrier to uptake.MethodsA quasi-experimental, pre-post intervention clinical trial without a control group was conducted to assess the immunogenicity and safety of intradermal (ID) JE vaccine. Healthy adults (18-45 years) received one dose of 0.1 mL (20% of standard dose) ID Imojev® (JE live attenuated chimeric vaccine, Sanofi-Aventis). Adverse events following immunization (AEFIs) were recorded 10 days post-vaccination. Blood samples were collected at baseline, 4 and 8 weeks post-vaccination. Neutralizing antibodies were measured using 50% plaque reduction neutralization test (PRNT50). Seroconversion was defined as PRNT50 titre ≥10. An in vitro study was also conducted to quantify the rate of decay of vaccine potency after reconstitution.ResultsIn total, 51 participants (72.6% females, median age 31 years), all non-reactive to JE virus at baseline were enrolled. Mild and moderate AEFIs were reported by 19.6% of participants; none required medical attention or interfered with normal daily activities. All participants seroconverted at 4 weeks (GMT 249.3; 95%CI:192.8-322.5) and remained seropositive at 8 weeks (GMT 135.5; 95%CI:104.5-175.6). Vaccine potency declined at a rate of 0.14 log plaque-forming units/0.5 mL per hour.ConclusionsIn healthy adults, a single 0.1 mL ID dose of Imojev was safe and immunogenic, at least in the short term. Reconstituted vials of Imojev vaccine may not retain their potency after 6 hours. Fractional JE ID vaccination could be a cheaper yet effective alternative for short-term travellers. Further studies need to investigate the immune response in a wider age range of individuals and the long-term immunogenicity of fractional JE ID vaccines.Clinical Trials RegistrationACTRN12621000024842.© International Society of Travel Medicine 2022. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com.

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