Virulence variation and live attenuated vaccines of Japanese encephalitis virus in China

Chinese scientists have isolated hundreds of Japanese encephalitis virus (JEV) strains from nature and studied their virulence in mice since 1949. They found that the JEV isolates in nature had apparently different virulence among them, especially the pheripheral virulence. This difference was caused by the effects of virus living circumstance such as the mosquito vectors, hosts and natural climate etc. Further studies demonstrated that the virulence of JEV could be reduced in laboratory by serial passages of the virus in mouse brain or different cell cultures especially in hamster kidney cells and the pheripheral virulence was reduced more rapidly. The technique of virus plaque purification was able to differentiate the higher and lower virulent viruses and can be used for selection of avirulent virus strain.By using different ways several attenuated virus strains have been selected and used as live vaccine for vaccination in humans, pigs and horses. Especially, they found that in vivo passage of the viruses in non-neural tissues of laboratory animals could further reduce residual virulence and promote their immunogenicity. By using this creative technology, a highly attenuated, immunogenic and genetically stable attenuated strain SA14-14-2 virus was selected. The attenuated live vaccine made by SA14-14-2 strain has been used worldwild since its licensure in 1989. And in 2013, the vaccine made by Chengdu Biological Institute obtained WHO prequalification. Key words: Japanese encephalitis; Isolates; Virulence; Live attenuated vaccine