Japanese encephalitis (JE) is a vector-borne viral zoonosis that also affects humans. JE virus is a member of the vector-borne flaviviruses, with similarities to several other flaviviruses such as dengue fever and West Nile virus. JE occurs in practically all Asian countries. The major genotypes of the virus have different geographical distributions, but all belong to the same serotype and are similar in terms of host preference and virulence. Following an infectious mosquito bite, the virus replicates in local and regional lymph nodes which may be followed by viral invasion of the central nervous system. Infection is characterized by sudden onset of fever, chills, myalgias, and mental confusion. In children gastrointestinal pain and vomiting may be the dominant initial symptoms and convulsions are very common. Nearly 3 billion people are believed to be at risk for JE virus infection and approximately 20,000 clinical cases with 6,000 deaths are reported annually. The case fatality rate ranges from 5-30% but approximately 30-50 % of the surviving patients have permanent neuropsychiatric sequelae and complete recovery occurs in only one-third of patients.
Vaccination of humans is the most effective means of preventing JE. There are three types of inactivated vaccines and one type of live attenuated vaccine currently used in the world:
- mouse brain-derived, purified vaccine, which is based on either the Nakayama-NIH or Beijing-1 [P-1] strains;
- primary hamster kidney (PHK) cell-derived, purified vaccine, based on the Beijing-3 [P-3] strain;
- Vero-cell derived purified vaccine based on the P-1, P-3 or SA14-14-2 strains as virus seeds.;
- PHK cell derived live attenuated vaccine based on the SA14-14-2 strain of the JE virus.
JE vaccine standardization
Japanese Encephalitis Vaccine (inactivated)
Requirements for inactivated Japanese Encephalitis vaccines produced in primary hamster tissue cultures or mouse brains were adopted in 1987. This guidance was updated in 2007 and the scope extended to included vaccines produced in a continuous Vero cell line.
Recommendations for Japanese Encephalitis Vaccine (inactivated) for human use
Recommendations for Japanese encephalitis vaccine (inactivated) for human use (Revised 2007), TRS 963, Annex 1
Japanese Encephalitis Vaccine, live attenuated
Guidelines for a live attenuated JE vaccine produced in primary hamster kidney (PHK) cells using the SA 14-14-2 strain of virus were adopted by the Expert Committee on Biological Standardization in 2000.
Recommendations to assure the quality, safety and efficacy of Japanese Encephalitis vaccine (live, attenuated) for human use
No WHO reference materials for JE vaccines are currently available.
WHO Informal consultation on scientific basis of specifications for production and control of Japanese Encephalitis vaccines (inactivated) for human use, WHO, Geneva, 1-2 June 2006