Serological assays as alternatives to the Ph Eur challenge test for batch release of tetanus vaccines for human use

According to the European Pharmacopoeia (Ph Eur) monograph on Tetanus Vaccine (adsorbed) (1997: 0452), assessment of potency is based on a challenge test in guinea pigs or mice. The end-point is taken as paralysis or death. The test requires a large number of animals and causes severe distress. The aim of the present study was to refine the test, and reduce the number of animals needed, for batch release purposes. Serological assays having the potential of being internationally accepted, have been compared with Ph Eur assays. The study included five tetanus vaccines of various combinations and produced by different manufacturers. The results indicated an excellent correlation between enzyme-linked immunosorbent assay (ELISA) and the challenge test (about 93% predictive value), as well as between the toxin binding inhibition (ToBI) test and the challenge test (about 95% predictive value) and between ELISA and the ToBI test (r = 0.92). Antitoxin concentrations determined by ELISA and ToBI were generally in the same range. An overall good correlation was also seen for serum pools of the guinea pigs injected with equal vaccine doses, between toxin neutralisation test in mice (TNT) and ELISA (r = 0.93) as well as between TNT and ToBI (r = 0.97). The ultimate goal of this project was to determine whether serological assays can be used for testing combined vaccines, particularly for tetanus and diphtheria components, using sera from the same animals.