Efficacy of single-dose SA 14-14-2 vaccine against Japanese encephalitis: a case control study

Background: In China, since 1989, an estimated 120 million children have be en immunised with the SA 14-14-2 live-attenuated Japanese encephalitis (JE) vaccine at ages 1, 2, and 6 years. A case-control study of licensed vaccin e found two doses to be 98% effective. Subsequently, researchers found that single-dose vaccine efficacy was high; we aimed to confirm this result. Methods: During July 11-24, 1999, 160 000 doses of JE vaccine were given to children aged 1-15 years, resident in three districts of Nepal. Several ca ses of JE were admitted to hospital from early August. We obtained names an d addresses of cases with serological evidence of a recent infection from B heri Zonal Hospital, Nepalgunj. We did a matched case-control study and cal culated the odds ratio of vaccination among JE cases and age-sex matched vi llage controls. Findings: 20 children, aged 1-15 years, were identified whose illness confo rmed with the JE case definition and were resident in villages receiving th e vaccine. None of 20 JE cases had received JE vaccine compared with 326 of 557 age-sex matched village controls. The efficacy of a single dose of JE vaccine was 99.3% (CI 94.9-100%). Interpretation: A single dose of JE vaccine is highly efficacious in preven ting Japanese encephalitis when administered only days or weeks before expo sure to infection.