A Vero cell method for potency testing of diphtheria vaccines

A collaborative study on the evaluation of an alternative functional assay to the Ph. Eur. in vivo challenge procedure for potency determination of diphtheria toxoid vaccines was initiated in January 2001. This study is an extension of the collaborative study for the validation of serological methods for potency testing of tetanus toxoid vaccines for human use. The primary aim was to examine whether a Vero cell assay is a valid alternative to the Ph. Eur. in vivo challenge methods for potency determination of the diphtheria toxoid component of combined vaccines. The study also investigated whether sera from the same immunised guinea pigs can be used for potency determination of both the diphtheria and the tetanus toxoid components. The project was divided into three parts. In part I, summarised in this publication, the operating procedures for the methods and the optimal vaccine dilutions for the immunisation of guinea pigs were established. From the results of the part I study, performed in two laboratories, the correlation co-efficients between the Vero cell method and the challenge assay were considered satisfactory to warrant continuing with the next phase. Comparable diphtheria potency estimates were obtained in the two assays for four vaccines of different activities (range ca. 20-200 IU/ml). The study also provided preliminary information that sera from the same guinea pigs may also be used for potency determination of both diphtheria and tetanus toxoid components of vaccines by ELISA.