[Intravenous inoculation direct challenge method for potency test of Japanese encephalitis vaccine on suckling mice].

The potency test of Japanese encephalitis (JE) vaccine is routinely carried out by intracerebral challenge of immunized mice using the Nakayama strain of JE virus, however since natural infection is by the peripheral route with a wild strain, one of the authors has reported (Bull. WHO, 43, 553, 1970) a new method by a subcutaneous challenge test on suckling mice. Since the usual route of infection is the result of mosquito bite, the authors developed a new challenge test by the intravenous route. Suckling mice 9-13 days old were immunized twice intraperitoneally with 0.15ml of different serial dilutions of vaccine on day 1 and 4. A challenge dose of JE virus equal to 500 icLD50 for suckling mice was administered into the tail vein on day 8, when the mice were not more than 20 days old. The JaGAr-01 strain was used for the challenge virus as it had been more recently isolated and is more virulent than the Nakayama strain. This method should give a more accurate assessment of the effect of vaccine the purpose of which is to protect humans from infection.