A novel approach for estimation of anti-FMDV protective immunity generated in multiple vaccinated field animals

In this study, forty serum samples from field buffaloes vaccinated with inactivated foot-and-mouth disease (FMD) vaccine were collected. These animals were multiple vaccinated with the above vaccine during previous years. The study was conducted to determine the actual status of the protective antibodies generated after vaccination. Initially, the serum samples were tested by Liquid phase blocking ELISA (LPBE), and only samples with titer more than 1.4 in LPBE were chosen for further analysis. These samples were tested with an in-house Gold Nanoparticle-based test for detection of anti-FMDV structural protein antibodies, in which the antibodies were detected at 10-4 dilution; this was suggestive of strong antibody titer generated post-vaccination. To test the binding affinity of these antibodies with the antigen, an avidity ELISA was developed and outcomes were expressed in terms of avidity index (AI). It was found that the avidity was low in some of the animals even after multiple vaccinations. Therefore, multiple vaccinations and strong antibody titer generation may not be the actual indicator of the protective immune response generated. We conclude that avidity ELISA can be a better approach than LPBE to measure the level of protective antibodies generated post-vaccination.