CONCLUSION

Concomitant hatch-day administration of Cevac IBird and Cevac Vitabron L provided meaningful protection against challenge with a Thai QX-like IBV strain in commercial broilers. Vaccinated birds achieved a CPS of 58.5% and a BPS of 100%, whereas the unvaccinated challenged group showed complete ciliostasis, with a CPS and BPS of 0%. Vaccination also reduced the proportion of RT-PCR-positive birds from 10/10 in the positive control group to 7/10, although this difference was not statistically significant. In addition, vaccinated birds developed higher antibody titers after challenge, confirming effective immune priming. However, vaccination did not prevent the short-term reduction in body weight associated with challenge, as both challenged groups had lower body weights than the negative control group at 5 dpc.

These findings demonstrate that concomitant administration of antigenically distinct live vaccines can preserve tracheal ciliary function and enhance immune responses against heterologous Thai QX-like IBV challenge. The strategy is operationally suitable for hatchery use and may provide a practical alternative to sequential vaccination programs in commercial broiler production. Nevertheless, the partial viral detection and absence of protection against early growth depression indicate that the vaccine combination did not provide complete protection. Larger field trials with longer follow-up periods, quantitative assessment of viral load and shedding, and evaluation against multiple circulating IBV genotypes are required. Overall, concomitant hatch-day vaccination represents a promising component of integrated IBV control programs, provided that vaccine selection is guided by continuous surveillance of locally circulating strains.