Protection conferred by a vaccine derived from an inactivated Egyptian variant of infectious bronchitis virus: a challenge experiment

The current study investigated the protective efficacy of a formalin-inactivated infectious bronchitis virus (IBV) vaccine derived from the field strain KP729422, which exhibits low S1 spike protein sequence homology (77.1–79.8%) with the currently used vaccine strains in Egypt. Two-week-old, specific-pathogen-free chickens were subcutaneously inoculated with a single dose of the vaccine containing 10 6.7 50% embryo infective dose (EID 50 ) of the inactivated virus. At 6 weeks of age, the chickens were challenged with 10 4 EID 50 of the same virus strain via the oculonasal route. In comparison with the unvaccinated challenged group, the vaccinated chickens had significantly higher IBV-neutralizing antibody titers and exhibited efficient protection against challenge on the basis of tracheal ciliary activity. However, the challenge virus was recovered from the kidneys and tracheas of these chickens at rates of 40% and 60%, respectively. These findings suggest that a single application of the vaccine may provide sufficient clinical and respiratory protection, but may not ensure complete protection against infection by the challenge virus.