A validation study has been performed to determine the suitability of the toxin binding inhibition (ToBI) test for the serological estimation of the potency of the
tetanus toxoïd component in
vaccines. 37 Murine serum pools over a wide range of antibody levels were titrated in both toxin neutralization (TN) and ToBI test. A good correlation was found between both assays. Sixteen DPT-
polio, twelve DT-
polio and seven T
vaccines were tested in the mouse lethal challenge test and the in vitro serological test, using the ToBI test for determining
vaccine-induced
tetanus antibodies. For all three types of
vaccine a statistically valid correlation between both assays was found. However, for two batches of DPT-
polio vaccine an "overestimation" of the
tetanus potency was observed in the serological assay compared to the challenge assay. This phenomenon could not be explained by the difference in immunization period nor by misinterpretation of the ToBI test of DPT-
polio-induced
antibodies. In the LPF test high LPF activity was observed for the deviating DPT-
polio vaccines. Therefore, the effect of
pertussis toxin (PT) on the potency of the
tetanus component in the serological assay was examined. The addition of 2 micrograms of PT to a "normal" DPT-
polio vaccine resulted in a nearly twofold increase of the
tetanus potency. It was concluded that
pertussis toxin has a
vaccine dose-dependent adjuvant effect on the potency of
tetanus toxoïd resulting in high potency values when determined by ToBI procedure. It is unclear how these findings should be interpreted with respect to the behaviour of such
vaccines in man.