Safety:

To describe the safety profiles following vaccination.

Immunogenicity:

To describe the immune response after a single dose of vaccine.

This is a randomized, cross-over, open, active controlled, multi-center trial in toddlers and children in Thailand. Stepwise enrollment of children in 2 age cohorts. Subjects will be followed for 5 years.

Inclusion Criteria:

• Provision of consent form signed by at least one parent or another legally acceptable representative, and by at least one independent witness.
• Completion of vaccinations according to the national immunization schedule
• Subject and parent/legally acceptable representative able to attend all scheduled visits and comply with all trial procedures.
• Previous receipt of two doses of a mouse-brain-derived JE vaccine at 12 to 15 months of age according to the national immunization schedule and aged 2 to 5 years on the day of inclusion.
• Aged 12 to 24 months on the day of inclusion and have not received any JE vaccine.

Exclusion Criteria:

• Participation in another clinical trial investigating a vaccine, drug, medical device, or a medical procedure in the 4 weeks preceding the first trial vaccination.
• Known or suspected congenital or acquired immunodeficiency, immunosuppressive therapy such as anti-cancer chemotherapy or radiation therapy within the preceding 6 months, or long-term systemic corticosteroids therapy.
• Known systemic hypersensitivity to any of the vaccine components or history of a life-threatening reaction to the trial vaccine or to a vaccine containing any of the same substances.
• Chronic illness, at a stage that could interfere with trial conduct or completion, in the opinion of the Investigator.
• Receipt of blood or blood-derived products in the past 3 months, that might interfere with the assessment of the immune response.
• Receipt of hepatitis A vaccine.
• History of flavivirus infection (confirmed either clinically, serologically or microbiologically).
• Administration of any anti-viral within 2 months preceding the screening visit.
• History of central nervous system disorder or disease.
• Personal or family history of thymic pathology (thymoma), thymectomy, or myasthenia.
• Planned participation in another clinical trial during the present trial period.
• Receipt of any vaccine in the 4 weeks preceding the first trial vaccination.
• Planned receipt of any vaccine in the 4 weeks following any trial vaccination.
• Personal human immunodeficiency virus, hepatitis B or hepatitis C seropositivity in the blood sample taken at screening.
• Thrombocytopenia, bleeding disorder or anticoagulants in the 3 weeks preceding inclusion contraindicating IM vaccination.
• Previous vaccination against flavivirus disease at any time before the trial other than a mouse-brain-derived JE vaccine given in a two-dose regimen at 12 to 15 months of age in accordance with the national immunization schedule.
• Febrile illness or any acute illness/infection on the day of vaccination, according to investigator judgment
• History of seizures.
• Previous vaccination against flavivirus disease.