Studies on the minimal number of animals required to achieve assurance of satisfactory potency in diphtheria and tetanus vaccines.

The purpose of the potency test for adsorbed diphtheria and tetanus vaccines prescribed in the European Pharmacopoeia is to provide 97.5% assurance that the minimum requirement for potency is exceeded. In order to achieve this the use of at least six groups of sixteen guinea pigs or mice is prescribed. In some formulations, particularly of combined vaccines, the potency exceeds the minimum requirements by a very large margin and the numbers of animals used in the test appear to be unnecessarily large. Data are presented which show that 97.5% assurance of satisfactory potency can be consistently achieved with substantially smaller numbers of animals than are used at present. It is suggested that the prescription of large numbers of animals in routine issue tests for diphtheria and tetanus vaccines is unnecessary and inappropriate except in cases where the potency of the vaccine is in dispute.