Investigation of an aerosol challenge model as alternative to the intracerebral mouse protection test for potency assay of whole cell pertussis vaccines.

The current potency test for whole cell pertussis vaccines, the intracerebral mouse protection test, is still the only assay which has shown a correlation with protection in children. However, it has considerable disadvantages as it uses a severe challenge procedure and the results tend to show significant intra- and inter-laboratory variation. An alternative assay based on non-lethal aerosol challenge of mice has been investigated as a replacement for the current intracerebral mouse protection test. Evaluation of this indicated that the aerosol system allowed consistent inoculation of bacteria into mice and gave good reproducibility. The protective capacity of different vaccine preparations was distinguished by this assay. Furthermore, the viable counts of Bordetella pertussis in the lungs of challenged mice were immunisation dose-dependent, which allowed the relative potency of vaccines to be calculated. Comparison of potency of five batches of vaccine from different manufacturers assayed by both the intracerebral and the aerosol challenge methods ranked the vaccines in identical order. The results suggest that this method has potential for use as a potency test for whole cell pertussis vaccine which would result in a great reduction in the number of animals used. It would also replace the lethal challenge by a non-lethal procedure and thereby avoid the use of the severe intracerebral challenge procedure.