Characteristics of serology-based vaccine potency models for foot-and-mouth disease virus

Foot-and-mouth disease (FMD) vaccine potency testing involves hundreds of animals each year. Despite considerable efforts during the past decades, a challenge-free alternative vaccine potency test to replace the European protective dose 50% test (PD(50)) has not been implemented yet. The aim of the present study was to further characterize the properties of serological vaccine potency models.

Logistic regression models were built for 5 serological assays from 3 different laboratories. The serum samples originated from 5 repeated PD(50) vaccine potency trials with a highly potent A/IRN/11/96 vaccine. Receiver Operating Characteristic analysis was used to determine a serological pass mark for predicting in vivo protected animals. Subsequently, an estimated PD(50) was calculated and the serotype dependency of the logistic models was investigated.

Although differences were observed between the laboratories and the serological assays used, the logistic models accurately predicted the in vivo protection status of the animals in 74-93% of the cases and the antibody pass levels corresponded to 84-97% of protection, depending on the serological assay used. For logistic models that combine different serotypes, the model fit can be increased by inclusion of a serotype factor in the logistic regression function.

The in vitro estimated PD(50) method may be at least as precise as the in vivo PD(50) test and may accurately predict the PD(50) content of a vaccine. However, the laboratory-effect and the serotype-dependency should be further investigated.