The authors aimed to evaluate the safety and immunogenicity of a live attenuated SA14-14-2 Japanese Encephalitis
(JE) vaccine in healthy Thai infants. One hundred and fifty subjects aged 9-15 months were vaccinated with one dose of this
vaccine. Regarding the vaccine safety, during the 28-day post-vaccination follow-up, no vaccine-related serious adverse
events were reported. In terms of immunogenicity, the sero-conversion rate of a single dose vaccination was 95% (95% CI,
90.0-97.6%) within 90 days after vaccination and the geometric mean titer (GMT) was 66.1. Eight subjects with JE seronegative
on days 28-35 post-vaccination became sero-positive on day 90. Seven subjects who remained sero-negative during
days 28-35 and day 90 post-vaccination were successfully sero-converted after receiving a second dose 3 months later. Thus,
two doses of this JE vaccine resulted in a 100% (95% CI, 97.3-100%) sero-conversion rate with the GMT of 260.8. Eight
children with GMTs lower than protective level after a single vaccination demonstrated a booster response with GMT of 1237
after the second dose of JE vaccination.

In conclusion, the live attenuated SA14-14-2 vaccine was safe, well tolerated and highly immunogenic with 95%
and 100% sero-conversion rate after one and two doses, respectively. Nevertheless, its long-term immune response and
possible influences from natural dengue infection requires further evaluation.

Keywords: Japanese encephalitis, Vaccine, Immunogenicity