A potency assay for Western equine encephalomyelitis vaccine was developed which utilized mice as the test animal instead of guinea pigs or hamsters. By immunizing several groups of mice with dilutions of the vaccine and challenging them intracerebrally with virulent virus, it was possible to determine mathematically a dose of vaccine capable of protecting 50% of the animals (ED50). When log dilutions of virulent virus were used to challenge mice which were immunized with dilutions of the vaccine, there was no difference among the ED50 values for the dilutions of challenge virus. In a direct comparison of ED50 values determined from the immunization of mice and those determined from the immunization of guinea pigs, there were no differences in the rankings of the vaccines.
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