Abstract: A detection of protective titers of a bivalent live IBD vaccine was conducted using SPF chicks. The chicks were vaccinated at 14 days of age, and were bled and their blood samples and sera collected for the detection of neutralizing antibodies to IBDV in a virus-neutralization (VN) assay and simultaneously challenged with either a standard serotype Ⅰ virulent IBDV strain CJ801 or a virulent variant IBDV strain 1084A at 7, 8, 9, 10, 12, 14 days post-vaccination, respectively. A rectilinear regression equation between IBD antibody titers and protective rates was established. When the VN antibody titer (GMT, geometric mean titer for vaccinated chicks with the vaccine) increased by 2968, the vaccinated chicks had 89.21% and 82.59% protections against the challenges of strains CJ801 and 1084A, respectively. Consequently, the IBD antibody titer of 2968 (GMT) was tentatively regarded as a protective neutralizing antibody titer resulted from the vaccine against virulent IBD viruses.

Key words: Bivalent live infectious bursal disease vaccine, VN antibody titers, Protective rates