Seroepidemiology study of Japanese encephalitis neutralizing antibodies in southern Taiwan: a comparative study between urban city and country townships.

BACKGROUND: Japanese encephalitis (JE) occurs all over Asia, especially in southeastern regions. The aim of this study was to evaluate the current JE vaccination regimen in Taiwan by assessing the neutralizing antibody among people of various age groups from different living environments.
METHODS: From 1998 to 1999, 2365 (1016 male, 1349 female) students and home-visit volunteers were recruited from an industrialized city (Kaohsiung), and 712 (290 male, 422 female) students and volunteers from hospital outpatient and physical check-up units were recruited from country townships (Pintung County) for this study. Participants were between 6 and 74 years old. Serum JE neutralizing antibody was measured by the plaque reduction neutralization test with a 50% reduction as seroconversion. Incidence cases from 1971 to 1999 were collected from an active surveillance system run by the Center for Disease Control in Taiwan.
RESULTS: Seropositive prevalence peaked immediately after the second booster in first-grade students and declined as age increased. For those born before 1963, seropositive prevalence was significantly associated with age, sex, and living area. In the 1990s, the incidences of confirmed JE in Kaohsiung and Pintung County were 0.11 and 0.14 per 100,000 population, with only 2 and 1 patient age 10 years or younger, respectively. All but 2 patients had never been vaccinated.
CONCLUSIONS: Seropositive prevalence decreased gradually after vaccination. A third booster (the fifth shot) before age 18 years may further enhance the antibody titers, especially if the odds of natural infections have significantly reduced. Follow up studies on the changes of antibody titers over time among immunized populations are warranted in Asian countries where natural infections become less common.