A potency test which simulates natural exposure for measuring post-exposure activity of rabies vaccines. A proposal for preparing a relevant international reference preparation.

There is a basic difference between the mechanism of immunity at the pre- and post-exposure level. Due to the fact that vaccination precedes challenge in the available potency tests (Habel and NIH) they do not measure post-exposure activity of rabies vaccines. For these reasons a potency test in mice has been developed which measures post-exposure activity of rabies vaccines. In this test natural conditions have been simulated by using intramuscular inoculation of a street virus (fox salivary gland origin), killing approximately 50% of the control mice, whereas intraperitoneal inoculation with 0.5 ml of undiluted inactivated rabies vaccine within 24 hours after street virus infection, should protect all mice. A human rabies vaccine prepared in HDC and already licensed for post-exposure vaccination showed only some post-exposure activity in this newly developed potency test, whereas an experimental inactivated rabies vaccine protected all mice. The International rabies Reference Vaccine did not protect after exposure but, on the contrary, enhanced the rabies street virus infection (shorter incubation period and more mice died of rabies than in the controls); it is therefore unsuitable for measuring post-exposure potency. Authorities concerned with rabies vaccine control are invited to use this newly developed post-exposure potency test in their laboratories and if agreed upon its value, to prepare an adequate batch of inactivated tissue culture rabies vaccine which meets this test-requirement. Such a batch could be a candidate for the establishment of an international reference preparation measuring post-exposure potency of rabies tissue culture vaccines.