Development of a monoclonal antibody sandwich ELISA for the determination of antigen content and quality in diphtheria vaccines.

At present, quality control of diphtheria vaccines by both manufacturers and national control laboratories relies heavily on in vivo assays to confirm potency. As part of the VAC2VAC project we have developed a monoclonal antibody (mAb) enzyme-linked immunosorbent assay (ELISA) to measure the relative amount and quality of diphtheria toxoid (DTxd) in Diphtheria-Tetanus based vaccines, and believe this test has the potential to play a key role in a control strategy no longer including an in vivo potency test. The mAb ELISA is highly specific, has good dilutional linearity and is suitable for detecting DTxd in a range of different human vaccine products. We have demonstrated the ability of the assay to discriminate between batches of different content and quality using vaccine batches that had been prepared to contain differing amounts of DTxd or altered by exposure to heat or oxidative stress. We have also demonstrated successful transfer of the method to other laboratories and have shown that different diphtheria antigen materials may be able to serve as a reference antigen for local standardisation of the method. The assay is ideally suited for incorporation into a consistency approach for routine diphtheria vaccine quality control testing and may be suitable to serve as the stability indicating test in replacement of the current in vivo potency test.