Confidence in indirect assessment of foot-and-mouth disease vaccine potency and vaccine matching by percentage of expected protection.

MATTION, NORA; ROBIOLO, BLANCA; MARADEI, EDUARDO; PEREZ BEASCOECHEA, CLAUDIA; PEREZ, ALEJANDRO,; SEKI, CRISTINA; SMITSAART, ELIANA; FONDEVILLA, NORBERTO; PALMA, EDUARDO; GORIS, NESYA; DE CLERCQ, KRIS; LA TORRE, JOSE

Viena, Austria

Congreso; FMDWeek2010. New tools and challenges for progressive control; 2010

European Commission for the Control of FMD

Introduction: There is a strong consensus worldwide on the necessity to replace experiments using live viral challenge. Indirect serological tests may be used to assess the potency of a vaccine provided that a statistical evaluation has established a satisfactory correlation between the results obtained by the in vivo potency test in cattle and the alternative test, using the relevant vaccine serotype. Materials and Methods: Assessment of vaccine potency and cross protection between two serotype A FMDV strains (A24/Cruzeiro and A/Argentina/01) was carried out through the Expected Percentage of Protection (EPP). Serum titers obtained by liquid phase blocking competitive ELISA (lpELISA) and virus neutralization (VNT) in 10 PPG trials of an A24/Cruzeiro vaccine, were interpolated into previously validated logit transformation curves that correlate PPG with serology. Results: The CV% for intra-indirect potency repeatability were ¡Ü20% in the 10 trials.  The CV% for inter-indirect potency test reproducibility was ¡Ü4.32 % for lpELISA EPP and ¡Ü9.0% for VNT EPPs.  The variation of EPP by lpELISA was lower than the variation obtained in PPG (CV< 10.4%) for the same A24/Cruzeiro vaccine. VNT falsely rejected the vaccine batch in one out of ten occasions. According to PPG and EPP, A/Arg/01 strain was not cross protected by A24/Cruzeiro in any trial. The variability of all cross-protection assays was high, and dependent on which correlation curve (homologous or heterologous) was used for interpolation of the serum titers. However, in most outbreaks, a correlation curve for the emerging strain would not be available.  Discussion: The results strongly support the replacement of potency challenge tests by indirect serological assays, at least for A24/Cruzeiro FMDV strain.  While determination of EPPs by lpELISA titers showed an excellent repeatability, reproducibility and concordance with PPG for vaccine potency, assessment of cross-protection by VNT EPPs was closer to the PPG outcome.