Correlation of 140S antigen dose with the serum neutralizing antibody response and the level of protection induced in cattle by foot-and-mouth disease vaccines

An analysis was made of data from potency tests on fifteen batches of monovalent foot-and-mouth disease vaccine, comprising five batches each of type O, type A and type C. Regressions were calculated for the relation of percentage protection (probit) versus log 140S antigen dose and for the serum neutralizing antibody titre (log SN 50) versus log 140S antigen dose. Type O vaccines required a far higher level (220 ng) of 140S antigen to achieve a 50% protection level (PA 50) in cattle than did type A (2.4 ng) and type C (4.36 ng) vaccines. Type O antigen, dose for dose, was as effective at provoking neutralizing antibody as the types A and C antigens. Thus, it would appear that a far higher log SN 50 value (2.14) was required for type O vaccines to equate with 50% protection of cattle than was required for type A (1.17) and type C (1.41) vaccines. Prior to 1977, however, the PA 50 value for type O vaccine strain was only 1.34 and it was concluded that an antigenic shift was the most likely cause for the large difference between that value and the current PA 50 value.