International collaborative studies on the pertussis vaccine potency assay. Part played by the challenge in the mouse-protection test.

Because of wide variations in the methods used to assay the potency of pertussis vaccine, the effect of the type of challenge used in the mouse-protection test was studied. Eight laboratories took part in a collaborative investigation and the potencies of 4 pairs of vaccines were estimated. Two methods were used, a "local method" using local strains and procedures for challenge, and a "study method" using a distributed strain and a specified procedure. For both methods the challenge level (mouse LD(50)) and the number of colony-forming units per challenge dose varied greatly between laboratories.The results of the collaborative assay show that the use of a common challenge, Bordetella pertussis strain 18-323, and a uniform method for preparing the challenge did not reduce the heterogeneity of the results. Other factors influencing the results of potency assays, such as the use of different strains of mice and the effect of levels of challenge, have still to be investigated.