Japanese encephalitis (JE) is the most important form of viral encephalitis in Asia. It is estimated that the JE virus causes at least 50 000 cases of clinical disease each year, mostly among children aged less than 10 years, resulting in about 10 000 deaths and 15 000 cases of long-term, neuro-psychiatric sequelae. In recent decades, outbreaks of JE have occurred in several previously non-endemic areas. Infections are transmitted through mosquitoes that acquire the virus from viraemic animals, usually domestic pigs or water birds. Only about 1 in 250-500 infected individuals manifest clinical disease. There is no specific antiviral treatment for JE. Although the use of pesticides and improvements in agricultural practices may be contributed to the reduction of disease incidence in some countries, vaccination is the single most important control measure. Currently, the three types of JE vaccines in large-scale use are (i) the mouse brain-derived, purified and inactivated vaccine, which is based on either the Nakayama or Beijing strains of the JE virus and produced in several Asian countries, (ii) the cell culture-derived, inactivated JE vaccine based on the Beijing P-3 strain, and (iii) the cell culture-derived, live attenuated vaccine based on the SA 14-14-2 strain of the JE virus. Drawbacks of the mouse-brain vaccine are the limited duration of the induced protection, the need for multiple doses, and, in most countries, the relatively high price per dose. The cell culture-derived vaccines are manufactured and widely used in China, where the inactivated vaccine is gradually being replaced by the live attenuated vaccine. Several other promising JE vaccine candidates are in advanced stages of development.
The need for increased regional and national awareness of JE and for international support to control the disease is urgent. JE vaccination should be extended to all areas where JE is a demonstrated public health problem. The most effective immunization strategy in JE endemic settings is a one time campaign in the primary target population, as defined by local epidemiological data, followed by incorporation of the JE vaccine into the routine immunization programme. This approach has a greater public health impact than either strategy separately.