Updating of the correlation between lpELISA titers and protection from virus challenge for the assessment of the potency of polyvalent aphtovirus vaccines in Argentina

Routine vaccination campaigns are carried out in Argentina twice a year, involving more than 100 million doses of foot-and-mouth disease (FMD) vaccine. Although the challenge test in cattle has not been totally replaced for the assessment of FMD vaccine potency, Argentine Animal Health authorities have used an indirect alternative method based on specific correlation studies of protection against podal generalization (PPG) tests performed in cattle with a validated liquid phase blocking ELISA (lpELISA). The change of vaccine formulations that took place after the 2000–2001 outbreaks, generated a gap in the correlation between lpELISA titers and PPG for the new FMD virus strains. A reappraisal of the correlation between lpELISA titers measured at 60 dpv and virus challenge by the PPG method at 90 dpv, performed for the four virus strains presently included in the Argentine vaccine is presented in this work. The data were obtained from 40 bovine challenge trials (647 sera) performed using exclusive batches of commercial vaccine from the year 2001 to January 2008 for A24/Cruzeiro, A/Argentina/2001, O1/Campos and C3/Indaial FMD virus strains. Curves of percentage of expected protection (EPP) versus lpELISA titers were obtained by logit regression for A/Argentina/2001, O1/Campos and C3/Indaial strains, but not for A24/Cruzeiro strain. The concordance between the direct and indirect tests using an EPP cut off value of 75% (82%, kappa = 0.62), in agreement with data originating from many years of vaccine control in Argentina, remarks the relevance of the acceptance of indirect alternatives to in vivo potency testing.