AN ATTENUATED MUTANT OF JAPANESE ENCEPHALITIS VIRUS.

In the course of studies towards the development of a live, attenuated Japanese encephalitis vaccine, an attenuated mutant (m mutant) of Japanese encephalitis virus was isolated after serial passages of a parental virus, Mukai strain, in embryonic mouse skin cultures. The low virulence of the m mutant was found to be stable in tissue cultures but reversion to virulence was observed after multiplication of the virus in mouse brain. The m mutant induced high immunity in mice after vaccination in a single intraperitoneal injection. Furthermore, a high virus yield was obtained from the m mutant in tissue culture, with titres of about 10(9.5) TCD(50) per ml. It is suggested that this mutant may be suitable for the preparation of inactivated tissue culture vaccine, but the author stresses that it cannot be recommended for tests in man until more is known about its genetic stability.