Protective efficacy of concomitant day-old vaccination with live-attenuated infectious bronchitis virus (IBV) vaccines (1/96 and H120 + PHY.LMV.42) against a Thai QX-like IBV challenge in commercial broilers.

Background and Aim: Infectious bronchitis virus (IBV) remains a major cause of respiratory disease and economic loss in commercial poultry production, particularly with the emergence and widespread circulation of QX-like variants. In Thailand, these strains continue to challenge existing vaccination programs because of their genetic diversity and limited cross-protection afforded by conventional vaccines. This study evaluated the protective efficacy of concomitant day-old administration of two live-attenuated IBV vaccines (1/96 and H120 + PHY.LMV.42) against challenge with a Thai QX-like IBV strain in commercial broiler chickens. Materials and Methods: Seventy day-old Cobb500 broiler chickens were randomly assigned to vaccinated, positive control, and negative control groups. Birds in the vaccinated group received concomitant ocular administration of the 1/96 and H120 + PHY.LMV.42 vaccines at hatch and were challenged at 21 days of age with a Thai QX-like IBV strain (10⁴ median embryo infectious dose [EID₅₀]/0.1 mL). Protective efficacy was evaluated using tracheal ciliostasis protection score (CPS), binomial protection score (BPS), reverse transcription-polymerase chain reaction for viral detection, serological responses determined by enzyme-linked immunosorbent assay, and body weight measurements. Statistical analyses were performed using one-way analysis of variance or the Kruskal-Wallis test, with p < 0.05 considered statistically significant. Results: Vaccinated broilers exhibited significantly improved tracheal protection compared with unvaccinated challenged controls, achieving a CPS of 58.5% and a BPS of 100%, whereas positive control birds demonstrated complete ciliostasis (CPS = 0%). Viral detection was reduced in vaccinated birds, although the reduction was not statistically significant. Vaccinated birds also developed significantly higher antibody titers after challenge than negative controls (p < 0.05), indicating enhanced humoral immunity. Body weight at 26 days of age remained significantly lower in both challenged groups than in unchallenged controls, suggesting that vaccination only partially mitigated the adverse effects of IBV infection on growth performance. No mortality occurred during the experimental period. Conclusion: Concomitant day-old administration of the 1/96 and H120 + PHY.LMV.42 live-attenuated vaccines provided measurable cross-protection against a Thai QX-like IBV challenge by preserving tracheal ciliary function, enhancing antibody responses, and reducing viral detection in commercial broilers. This hatchery-applicable vaccination strategy represents a practical approach for improving early protection against circulating QX-like IBV strains in Thailand. Nevertheless, protection remained partial, and additional studies involving larger populations, diverse circulating variants, quantitative viral load assessment, and commercial field validation are required to optimize vaccination strategies and confirm long-term protective efficacy.