Potency tests of a total of 658 foot-and-mouth disease (FMD) vaccine batches by bovine protective dose (PD50) showed that 22% of the tests did not follow the expected theoretical model and the results could not be interpreted because of the lack of a dose-response relationship. The protection against generalization (PG) method, using undiluted vaccine, proved to be highly reliable from the analysis of the results of 65 replicate potency tests of three hydroxide-saponin (HS) vaccines and two oil-adjuvanted (OL) vaccines. The PG results were consistent in 64 out of the total of 65 tests. The data obtained indicated that, with regard to direct tests in cattle, the PG test is the preferred method for FMD vaccine potency control. This study also showed that the incorporation of a reference vaccine in routine potency tests is essential to ensure normality of the test system and that good quality HS and OL FMD vaccines may maintain immunogenic stability for at least 27 and 36 months, respectively, when stored at 2-8 degrees C.
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