The use of serum neutralizing antibody assay for the determination of the potency of foot and mouth disease (FMD) vaccines in cattle.

The Permanent Commission of O.I.E. on foot and mouth disease has proposed a series of potency standards for foot and mouth disease vaccines. No totally in vitro assay has yet been developed to satisfy these requirements, and most Control Authorities require a potency assay to be carried out in cattle using challenge with virulent virus. There are various reasons why challenge tests should be replaced by serological tests, but a stumbling block to this is the requirement that the 50% protective antibody level (PA50) is valid only for a specific combination of vaccine virus seed lot with cattle virus seed lot in a laboratory. The results of challenge and antibody tests for 38 U.K. Control Authority vaccine batch tests were analysed to examine the correlation between potencies measured by the two methods. The correlation between the methods was high and the proportion of misclassification of batches was low. It is suggested that the results provide a good basis for future use of the serological method, but that it is still sometimes necessary to carry out challenge tests.