Diptheria, Tetanus, Pertussis and combined vaccines
Diphtheria and Tetanus vaccines are amongst the most frequently used vaccines worldwide and have been remarkably successful products. Their use has resulted in a significant decrease in disease incidence in the industrialized world as well as in developing countries. Nevertheless, difficulties exist in the global harmonization of potency testing procedures, even when International Standards are used, and different approaches have been taken by different countries. Some follow WHO and European Pharmacopoeia procedures, whilst others follow the US procedures, with or without modifications. The approach taken by the European Pharmacopoeia, like that of WHO, is based on the determination of the immunizing potency of each final bulk by comparison with an appropriate reference material calibrated against the International Standard for Diphtheria Toxoid (adsorbed) or the International Standard for Tetanus Toxoid (adsorbed), as appropriate. There has been much activity in recent years in simplifying the current tests, to reduce the number of animals and to refine the end-point used in potency testing. Some studies have also considered the use of the same animals to test the potency of several antigens.
Despite many attempts to harmonize potency requirements globally, there are still no universally accepted methods. This leads to problems in international exchange of these vaccines due to difficulties in the mutual recognition of testing results. With the development of new combination vaccines, the need for harmonization of the Diphtheria and Tetanus potency tests has increased, creating an unique opportunity to resolve this longstanding issue.