An assessment of the interval between booster doses of Japanese encephalitis vaccine in the Torres Strait

Objective: Japanese encephalitis (JE) emerged for the first time in the Torres Strait, north Australia, in 1995. The inactivated mouse-brain derived JE vaccine was offered to all residents of the outer Torres Strait Islands prior to the 1996 wet season. This study was undertaken to determine the appropriateness of the recommended three-year interval between booster doses of the vaccine. Methods: JE neutralising antibody was measured in residents of Badu Island for whom 30-36 months had passed since either a previous booster or the completion of the primary immunisation series. Results: Only 70 (32%) of 219 eligible individuals had protective antibodies; 50 (37%) of the adults were immune, compared with 20 (24%) of the children (odds ratio (OR) 1.93; 95% confidence interval (Cl) 1.01-3.74). Conclusions: This low level of immunity suggests that there is little in the way of natural boosting from either JE or other closely related viruses. Given the apparent low level of risk of exposure to the JE virus in the Torres Strait, and the logistical complexities involved in delivering the booster doses, the current recommendation of a three-year interval is not inappropriate. Implications: It would be advantageous to have a JE vaccine that is not only safer but also more immunogenic, so that it might be possible to further increase the booster dose interval.