Japanese encephalitis (JE) is the main cause of viral encephalitis in many countries of Asia including Thailand. Estimated annual mortality ranges from10,000-15,000 deaths, while the total number of clinical cases is about 50,000. Of these cases, about 50% result in permanent neuropsychiatric sequelae. The disease occurs mostly among children aged <10 years. There is no specific antiviral treatment for JE. Vaccination is the single most important control measure. This study aims to evaluate the immunogenicity and safety of inactivated Vero cell derived JE vaccine (Beijing P-3 strain) produced by Liaoning Cheng Da Biotechnology Co., Ltd, China "JEVAC" in Thai children.

152 healthy Thai children aged between 1-3 years will be vaccinated with "JEVAC" in a dose of 0.5 mL. subcutaneously on Day 0, 1-4 weeks later and a booster vaccination at one year (totally 3 doses). Two mL. of blood will be drawn on Day 0, 4 weeks after second dose, at one year on booster vaccination day and 4 weeks after the booster (totally 8 mL. of 13 months study period) for determination of JE neutralizing antibodies (PRNT50) using Beijing P3 strain. Adverse events will be observed for 28 days after each vaccination. Serious adverse events will be observed throughout the study period.

Inclusion Criteria:

1. Healthy Thai children aged 1- 3 years
2. No previous history of JE vaccination
3. Available for all visited schedule in the study period.
4. Written inform consent signed by a parent or guardian

Exclusion Criteria:

1. Known serious underlying diseases such as nervous system, heart, kidney and liver diseases.
2. Known hypersensitivity to JE vaccine composition such as human albumin, dextran 40, etc.
3. Previous history of JE disease.
4. Receive the blood component within the past 3 months,
5. Known history of immunocompromised conditions such as HIV/AIDS, malignancy.
6. Under treatment of immunosuppressive drugs such as systemic corticosteroid and anti-neoplastic drug.
7. Febrile illness (temperature ≥37.5°C) or acute illness/infection on the day of vaccination
8. Plan to leave the study area before the end of study period.
9. Participating in other clinical trials.

In addition to systematic assessment on Day 0-14 after each vaccination, Any unsolicited AE was accessed within 28 days after each vaccination. Any SAE was accessed through the study period (1 yr +28 days).