Abstract
Background:
Infectious bronchitis (IB), caused by the IB virus (IBV), is a highly contagious respiratory disease of poultry that causes widespread global economic losses within the poultry industry. The high mutation rate of IBV and continual emergence of new serotypes present a major challenge for disease control which is mainly focused on vaccination.

Aim:
The objective of the current study was to evaluate the efficacy of two commercially available vaccines, MEVACTM IB H120 and MEVACTM IB VAR 2, against challenge with a heterologous virus, IBV QX in specific pathogen-free chickens.

Methods:
Serological responses to IBV QX challenge were evaluated using Enzyme-Linked Immunosorbent Assay (ELISA) and hemagglutination inhibition (HI).

Results:
By day 35, all vaccinated groups showed a significant increase in mean antibody titers to IBV QX. At days 14 and 28, all vaccinated birds demonstrated the presence of HI antibodies against IBV QX. A reduction in viral shedding was observed in all vaccinated groups. All vaccinated groups exhibited a clear reduction in ciliostasis scores, indicating that the vaccines provided protection against IBV QX challenge. Histopathological analysis further confirmed the beneficial effects of vaccination in reducing tracheal damage, with all vaccinated birds exhibiting lower scores than the control group. The study demonstrates that a combination of variant (MEVACTM IB VAR 2) and classical (MEVACTM IB H120) vaccine strains provided cross-protection against IBV QX heterologous challenge.

Conclusion:
In so doing, the study contributes important insights to help guide the development of future vaccination programs in combating the threat that IBV poses to the poultry industry.

Key words: Infectious bronchitis, Infectious bronchitis virus, IBV QX Challenge, Vaccine efficacy, Poultry respiratory pathogen