Questions concerning the potency of rabies vaccine.

In pre-exposure immunization the protective effect of rabies vaccines can be correlated with their ability to stimulate the production of neutralizing antibodies and in post-exposure therapy also antibodies would appear to have a major role. Therefore, the potency of the vaccines may conveniently be compared by measuring their antigenicity in animals or by serum blocking tests. Nevertheless, most authorities will license, for human and veterinary use, only those vaccines which reach the required standards in the Habel or NIH tests, both of which depend upon the inoculation of more than 1 dose of vaccine followed by an intracerebral challenge. We have been involved recently in testing batches of vaccine imported into Britain for use in animals and man. Since the U.K. authorities have adopted the NIH test for this purpose we have taken the opportunity to compare some of these vaccines in the NIH test, in a modified NIH test in which the same amount of vaccine was given in a single dose, and in a test in which the production of serum neutralizing antibodies was measured. We have found that one dose of vaccine given on day 0 gave less protection and less neutralizing antibody than the same amount of vaccine given as two separate doses on day 0 and day 7. The greater effect obtained by inoculating the vaccine in two doses could thus be misleading and we suggest the adoption of a test in which a single dose is given.