The highly contagious Gammacoronavirus infectious bronchitis virus (IBV) causes infectious bronchitis in domestic fowl, resulting in significant economic losses to poultry industries worldwide. Live attenuated vaccines against IBV are generated by 80 – 100 serial passages of a field strain in embryonated eggs, balancing attenuation with the retention of immunogenicity. The molecular mechanism of attenuation is unknown and vaccines present a risk of reversion to virulence. Over passaging can result in reduced vaccine efficacy due to the loss of immunogenicity. Immunogenicity includes antigenicity, i.e presentation of epitopes to immune cells as well as the ability of the virus to deliver the antigen, defined by both viral replication and tropism. Using recombinant IBVs (rIBVs) we investigated whether increased antigenicity or increased viral replication could improve vaccine efficacy. Chickens were vaccinated with rIBV and homologously challenged three weeks later. Clinical signs and tracheal ciliary activity were asessed both post vaccination and post challenge. Increased antigenicity appeared to have minimal effects on vaccine efficacy. A single point mutation in a non structural gene that increased vaccine virus replication reduced vaccine efficacy. Our study indicates that immunogenicity of live attenuated vaccines is multifactoral and that the rIBV genetic backbone can impact vaccine efficacy.
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