Foot-and-mouth disease vaccine potency testing: determination and statistical validation of a model using a serological approach.

European foot-and-mouth disease vaccine manufacturers are required to quantify the efficacy of their product in accordance with the European Pharmacopoeia (EP). The method used most often to establish the potency of foot-and-mouth disease vaccines requires viral challenge of vaccinated cattle. Alternative approaches, such as challenge-free serological assessments have many advantages over existing methods and could be used if robust statistical models could be developed that related antibody titres to protection from challenge. Logistic regression analysis of data from two independent research laboratories, representing six of the seven main serotypes of FMD, permitted the parameterisation of these models and indicated that a significant relationship existed between antibody titre and probability of protection. Furthermore, no significant differences were observed in the parameters of logistic models fitted to different strains within the serotypes A, O, and SAT-3, or when strains from serotypes A, O, and Asia-1, or SAT-1 and SAT-3, were combined. However, significant differences in the model parameters did exist between different laboratories. Using these models a bootstrap analysis suggested that for vaccines that induced consistently high titres, as few as six to eight individual animals could be used to establish with confidence the minimum protective doses that would protect 50% of vaccinated animals. We conclude that a serologically evaluated truncated test that eliminates the need to virus challenge cattle is a credible alternative for quantifying vaccine potency.