The eighth report of the WHO Expert Committee on Rabies suggested that inactivated veterinary vaccines with a potency of less than 1.0 IU per dose, as measured by the NIH test, should not be licensed or released unless an adequately designed experiment has demonstrated a duration of immunity of at least one year in the species for which the vaccine is to be used.
The potency of live and inactivated vaccines should be ascertained at intervals after they have been distributed. Inactivated vaccine, even in liquid form, and lyophilized modified live-virus vaccines are relatively stable when stored under proper conditions. To verify that storage conditions are adequate, it is recommended that samples from the field that are approaching their expiry date be tested using the methods applied to newly manufactured products.
Minimum potency requirements for oral vaccines for immunization of wild animals have not been generally established, although the median effective doses (ED50) of various modified live-virus and recombinant vaccines are known.
To test the efficacy of candidate vaccines for oral immunization, sufficient numbers of target animals should be maintained under captive conditions, given the vaccine and challenged with the virus. The potency of the vaccines should be standardized to quantifiable levels (e.g. plaque-forming units/ml, TCID50/ml). Once efficacy has been demonstrated in the target species under laboratory conditions, the vaccine should be administered in a bait identical to that to be used in field trials. Serial dilutions of test vaccine will determine the ED50. Animals should then be held for a minimum of 6-12 months prior to a challenge with a field strain administered by the intramuscular route; the interval between vaccine administration and challenge depends on the turnover rate of the target species. Potency estimates should not be based entirely on the ability of the vaccine to induce virus-neutralizing antibodies in the target species; environmental stability tests are also necessary to demonstrate that vaccine potency is retained under field conditions.