Abstract
Infectious Bursal Disease (IBD) is a viral, contagious immunosuppressive disease posing an important threat to the commercial poultry industry. Evolution of highly virulent strains of IBD virus warranted the need for detailed characterization of the immune responses offered by the currently available vaccines. Two extensively used live vaccines of varied attenuation levels - intermediate and intermediate plus - strains were analyzed for the induction of immune responses. Both the vaccines induced protective antibody titers with the onset, quicker and higher with the intermediate plus vaccine. The intermediate plus strain vaccinate was observed to induce higher levels of IFN-γ in the birds. These results were supported by immunophenotype analyses with an increase in CD8+ and simultaneous decrease in CD4+ cell population. Both vaccine strains conferred protective immunity against virulent challenge. The study warrants the use of intermediate plus vaccines in disease endemic regions and intermediate vaccines in non-endemic regions to prevent IBD infection.