Vero cell assay validation of an alternative to the Ph. Eur. diphtheria potency tests.

In the framework of the Biological Standardisation Programme of the European Pharmacopoeia Commission, in 1993 a collaborative study was organised for the validation of an alternative to the diphtheria in vivo challenge tests required by the Ph.Eur. monograph V.2.2.7. The alternative assay is based on the detection of neutralising antibodies in the sera from mice immunised with the vaccines to be tested (Vero cell assay). In the study this assay method was validated against intradermal and lethal challenge in guinea-pigs, performed in conformity with Ph.Eur. Therefore the potency currently on the European market, was assayed in parallel by the different assay methods. Seventeen laboratories, from eleven different countries, participated in the study. Three laboratories performed the intradermal challenge assay, while three other laboratories performed the lethal challenge assay. All seventeen laboratories performed the Vero cell assay. The results of the study suggest that the potency of the diphtheria component of both monovalent diphtheria vaccines and combined diphtheria-tetanus vaccines can be estimated adequately by means of the Vero cell assay. It does not yet seem possible for all combined diphtheria-tetanus-pertussis vaccines to replace a potency assay based on the protective capacity of a vaccine in guinea-pigs by the Vero cell assay. This may be due to an adjuvant effect of the pertussis component of the vaccine, in combination with the adsorbent used, which may be more pronounced in mice than in guinea-pigs and may also differ between different strains of mice.