Effect of single dose of SA 14-14-2 vaccine 1 year after immunisation in Nepalese children with Japanese encephalitis: a case-control study

Abstract

BACKGROUND: In July, 1999, a single dose of live-attenuated SA 14-14-2 Japanese encephalitis vaccine was given to children aged 1-15 years in the Terai region of Nepal. Cases of natural infection occurred almost immediately. Our aim was to assess the long-term protective effect of this vaccination. METHODS: In 2000, this same population had a second seasonal exposure to the virus. We therefore did a case-control study to measure the prevalence of vaccination against Japanese encephalitis in 35 patients hospitalised for the disease 1 year after immunisation, and in age-sex matched village controls. FINDINGS: Of 35 children resident in Bardiya and Banke districts admitted to the Bheri Zonal Hospital with serologically confirmed Japanese encephalitis, only one had been vaccinated in 1999. In 430 age-sex matched village controls, 234 (54.4%) were vaccinated. We calculated a median unbiased estimate of the odds ratio of 0.0155, with lower and upper confidence limits of 0.0004 and 0.0986. The protective effect of vaccine after 12-15 months was 98.5% (CI 90.1-99.2%). INTERPRETATION: Our study provides evidence of sustained high protection afforded by one dose of live attenuated SA 14-14-2 vaccine in Nepalese children.