Effectiveness of mouse brain-derived inactivated Japanese encephalitis vaccine in Thai National Immunization Program: a case-control study.

Mouse brain-derived, purified inactivated Japanese encephalitis vaccine (MB JEV) has been locally produced and introduced into Thai National Immunization Program (NIP) since 1990. However, MB JEV effectiveness has been concerned, since 30-40% of JE cases received JE vaccines were recently reported in two descriptive studies. Therefore, in 2010, a case-control study was designed to assess effectiveness of the MB JEV used in the NIP, among children aged 1 to <6 years. Subjects enrolled from all regions of the country, 26 cases were clinical encephalitis with laboratory-confirmed for JE IgM titer, while 103 controls had past illness free on encephalitis. For each case, four controls were enrolled from children living in the same community with the case. Individual health records of the subjects were used to verify JE immunization status. Among children aged ≥ 18 months old, the recommended age by the NIP, the effectiveness was estimated at 94.6% (95%CI, 79.6-98.6%) and adjusted effectiveness was 97.50% (95%CI, 88.60-99.50%). The study results suggest that the MB JEV used in the NIP is highly effective, even among children aged <18 months. Therefore, aggressive immunization using the MB JEV would greatly diminish disease burden.