The objective of this study is to measured the Geometric mean titer (GMT) of Japanese Encephalitis neutralizing antibody and proportion of seroprotection among the children who received a booster dose of JE-CV after the first dose of SA14-14-2 vaccine.

Study design: This open label clinical trial in 50 children aged 1-5 years, was conducted at King Chulalongkorn Memorial Hospital in Thailand. The protocol was approved by the Institutional Review Board of Chulalongkorn University, and the study was performed in accordance with Declaration of Helsinki, International Conference on Harmonization Good Clinical Practice, the European Directive 2001/20/EC, and written informed consent was obtained from parents or a legally acceptable representative before enrolment.

Vaccines: JE-CV was manufactured by Sanofi Pasteur Biologics Co., USA., and reconstituted using 0.4% sodium chloride diluent for injection; each dose 0.5 ml contained 4.0-5.8 log10 plaque forming units of virus Serology: JE neutralizing antibody levels were assessed using a PRNT50 assay. The final end point neutralization titer is the inverse of the highest serial dilution of serum that can neutralize ≥ 50% of JE challenge virus. Testing was performed at Focus Diagnostics Inc. using JE-CV as a challenge virus.

Statistical methods: sample size was calculated based on historical data from JE15 study, at month 24 after first dose of JE-CV, the GMT of JE neutralizing antibody was 39.4 (95% CI 33.7 to 46.0) and increase to 2242 (95% CI 1913, 2628) at day 28 post JE-CV booster dose. On the assumption that children who received SA14-14-2 vaccine and subsequently get one booster dose of JE-CV at 12-24 months later will have GMT of at least 1040, with 80% power and alpha 0.05, data at least 43 children need to be collected. When accounted for 15% of children who might loss to follow-up or cannot get adequate blood sample, 50 children should be enrolled.

The per-protocol population will be used for the main immunogenicity analyses. For the main parameters, 95% confidence intervals (CIs) of point estimates will be calculated using the normal approximation for quantitative data and the exact binomial distribution for proportions. The point estimates and their 95% CI of the following will be presented for each group of the Geometric Mean (GM) of neutralizing antibody on D0 and D28 and the percentage of subjects with neutralizing antibody >=10 at D0 and D28

Inclusion Criteria:

1 Children aged 1 to <5 years on the day of inclusion

2. History of received 1 dose of SA14-14-2 vaccine 12-24 months prior to enrollment

3. In good general health at the time of inclusion

4. Provision of informed consent by the parent(s) or legal guardian(s)

Exclusion Criteria:

1. Receipt of blood or blood products in the past 3 months.
2. Acute febrile illness on the day of vaccination.( BT > 38 C)