Chimeric cDNA for the expression of immunogenic polypeptides include the genetic epitopic determinants for a base infectious bursal disease virus strain and at least one other infectious bursal disease virus strain. The genetic epitopic determinants encode amino acids or amino acid sequences which define epitopes bound to by previously established monoclonal antibodies. The immunogens expressed by the cDNA may be employed to provide a vaccine against a plurality of IBDV strains. The epitopic determinant of IBDV lethal strains has been detected, and an immunogen for conferring immunity with respect thereto is disclosed. Similarly, a monoclonal antibody specific for IBDV lethal strains is identified, and a vaccine for passive immunization therewith is also disclosed. Immunogens exhibiting conformational epitopes, in the form of virus-like particles, are effective in the preparation of vaccines.