Cross-Protection by Infectious Bronchitis Viruses Under Controlled Experimental Conditions.

Infectious bronchitis virus (IBV) cross-protection trials were performed in healthy chickens maintained under controlled environmental conditions. Chickens primed or primed and boosted with a Massachusetts (Mass)-type attenuated vaccine were subsequently challenged with either IBV Arkansas (Ark) or GA13-type virulent strains. In addition, Ark-vaccinated chickens were challenged with IBV GA13. Spike protein 1 (S1) amino acid identities between IBV vaccine and challenge strains varied from 76.0% to 77.3%. Contrary to expectations, assessments of clinical signs, viral load, and histopathology indicated a significant level of cross-protection among these antigenically distant IBV strains. Moreover, prime and booster vaccination with Mass protected against GA13 and improved protection against Ark when compared with Mass single vaccination. These results emphasize the need to include both single vaccination control groups and control groups primed and boosted with a single serotype when testing the efficacy of IBV protectotypes and/or novel IBV vaccine combinations against heterologous serotypes under controlled experimental conditions. Such controls are of distinct importance in experiments supporting the introduction of attenuated IBV vaccine strains exotic to regions, since these exotic strains may provide new genetic material for recombination and emergence of novel IBV strains.