A new method for a potency test of Japanese encephalitis vaccine. Direct challenge method on suckling mice by subcutaneous inoculation

The potency of Japanese encephalitis (JE) vaccine is routinely tested by intracerebral challenge of immunized mice, but since natural infection is by a peripheral route the author was led to develop a subcutaneous challenge test, using the natural route. Suckling mice 8-10 days old were immunized twice intraperitoneally with 0.15 ml of different serial dilutions of vaccine on days 1 and 4. A challenge dose of JE virus equal to 5000 intracerebral LD(50) for suckling mice was given on day 8, when the mice were not older than 19 days. The JaGAr 01 strain was used as the challenge virus, being more freshly isolated than the Nakayama strain usually employed in Japan and therefore less likely to have been modified by repeated mouse-brain passage.This new assay method should give a more accurate assessment of the effect of a vaccine which is intended to protect man from natural infection only and not from infection by an unnatural route.