Comparison of Foot and Mouth Disease vaccine potency testing on cattle in terms of the nature of the diluent

Abstract

A study on more than 5000 cattle was carried out on the results of potency testing of Foot and Mouth Disease vaccines with a fixed challenge virus. These results are either taken from a series of publications, or obtained in our Institute. One thing which emerges clearly from this analysis is that, regardless of the author, one essential parameter must be considered: the mode of fractionation of the vaccine doses, characterizing three different biological systems: i) dilution using a buffer containing immune adjuvant substances (TA system or PD50 method); ii) dilution using a buffer containing no adjuvant substance (TC system or Pb method); iii) variable volume doses, with no dilution (VV system). Experimental validation in our Institute has been carried out on 3 vaccines controlled simultaneously with TC and TA systems. The TC system is 1.38 times more sensitive than the VV system and 2.60 times more sensitive than the TA system; this sensitivity is linked to the dose-response slope. Using an equal number of animals, the TC system is 1.69 times more precise than the VV system and 2.97 times more precise than the TA system. The yield data, expressed by the discriminating power for 15 animals, with a bilateral probability of 0.95 is 3.85 for TC, 9.8 for VV, and 53.1 for TA. These ideas are essential as soon as one wants to establish, based on experimental results, the minimum requirements of protection given by a vaccine according to the IOE, expressed by the D50. In order to guarantee an immediate minimum protection of 70 per cent with a unilateral probability of 1 − α = 0.95, with 3-level tests using 5 animals, we need 1.65 Pb in the TC system and 2.6 PD50 in the TA system. These minimum values are the lower confidence limits. Due to its precision, its sensitivity, its efficiency, and the fact that no adjuvant is used, the Pb method has been adopted by our Institute.