Dose-response effects of inactivated Newcastle disease vaccines: Influenceof serologic assay, time after vaccination, and type of chickens

Knowledge of the dose-response relation of inactivated vaccines and of the factors that influence this relation is essential for the evaluation of exi sting vaccine potency assays and the development of new potency assays that are based on the antigen content of the inactivated vaccines. We quantifie d the relation between vaccine dose, serologic response, and clinical prote ction after vaccination for three different inactivated Newcastle disease ( ND) vaccines. Qualitatively, similar dose-response curves were obtained for the three vaccines when either the serologic response or the clinical prot ection of specific-pathogen-free (SPF) chickens was plotted against the dif ferent vaccine doses applied. However, the vaccines differed quantitatively : doses of vaccines that induced similar antibody titers or clinical protec tion differed 2-8-fold. In contrast with the narrow range of antibody titer s induced by a full vaccine dose, a very broad range of titers was obtained after dilution of the vaccines. At least 95% of the SPF chickens with dete ctable antibody in the serum were protected against a challenge with virule nt Herts ND virus. The relation between the dosage of two different ND vacc ines and the serum antibody titers remained markedly constant between 3 and 18 wk after vaccination. Vaccination of broilers instead of layers with a dilution series of inactivated ND vaccine resulted in significandy lower an tibody levels and less clinical protection against virulent challenge. In c onclusion, despite quantitative differences, we found comparable dose-respo nse relations for the three inactivated ND vaccines studied.