A model for determining immunogenic relationships between avian infectious bronchitis viruses

A model for determining immunogenic relationships between strains of infectious bronchitis virus is described that is based upon the vaccinating dose required to induce prevention of multiplication of a standard challenge dose of an homologous strain in the lungs of specific-pathogen-free chickens. The model has been used to demonstrate that: (1) Approximately 100 times the vaccinating dose must be used to produce immunity from one compared with two doses; (2) differences of immunogenicity of greater than 1,000-fold exist between Australian strains; (3) immunogenicity is greatest for live virus administered directly to the trachea, followed by live virus administered by the ocular route, inactivated virus administered intratracheally and inactivated virus administered intramuscularly; (4) for chickens inoculated with Australian vaccine strains, protection against challenge is unrelated to serotype.