Virus content and effectiveness of foot-and-mouth disease vaccines.

(I) The amount of 22 nm particles in 26 batches of cattle tongue epithelium extract used for the preparation of C-type vaccine was determined with an improved 50% haemolytic and point complement fixation test after fluorocarbon precipitation of non-immunizing 7 nm particles. The total amount of 22 nm and 7 nm particles (alpha GN value) varied considerably from batch to batch, 22 nm components (alpha GF value) showing a maximum 5-fold difference. (ii) Effectiveness of vaccines with known virus content was tested in adult mice challenged with an adapted virus strain. In commercial C-type vaccines the complement-fixing activitiy of 22 nm particles and the potency of the vaccine showed a logarithmic regression (mouse index = -1.43 + 2.27 1g alpha GF).