Target alternative vaccine safety testing strategies for pertussis toxin

All acellular pertussis (aP) vaccines in use contain chemically inactivated pertussis toxin (PT). The finding that mice, naturally resistant to the effects of histamine, become sensitive upon injection of minute amounts of PT, led to the development of the test for residual PT known as the histamine sensitization assay (HSA). The HSA used by U.S.-licensed manufacturers is a limit test that shows that the residual bioactivity of PT in a single human dose of vaccine is below a threshold. Limit tests do not allow quantitative measurement. When the method is newly established at the point of use, three or more dilutions of pure PT are used to verify that mice injected with the vaccine came from a shipment that have sensitivity consistent with historical values. Sensitizability is expressed as an HSD₅₀ (the dose that sensitizes 50% of a group of mice). However, once linearity of the dose response has been demonstrated, the assay may be simplified so as to include in each test only a single control group injected with PT. This assay simplification constitutes an example of the so-called “consistency approach.” A Japanese variant of the HSA uses a drop in body temperature as a nonlethal alternative index of PT-mediated sensitization and can provide a quantitative estimate of the residual PT activity of a vaccine. However, the advantage of a quantitative method is not obvious, because the amount of PT that is unsafe for humans is unknown. In addition, although the use of a nonlethal endpoint constitutes an important refinement, the need for a reference group in the test to obtain a quantitative estimate increases the number of animals required, relative to the number used in a simplified limit test. Moreover, the nonlethal endpoint might be adapted to the limit test format, and important steps have been taken in this regard. Finally, one option under early evaluation is the possibility of using the results from two in vitro assays, an enzymatic activity assay and a binding assay, to replace the HSA.