INTRODUCTION

Infectious bronchitis (IB) is a highly contagious and economically important viral disease of poultry worldwide. It is caused by infectious bronchitis virus (IBV), a single-stranded RNA virus belonging to the genus Gammacoronavirus within the family Coronaviridae [1]. IBV primarily infects the respiratory tract but may also involve the renal and reproductive systems, resulting in substantial economic losses because of increased morbidity and mortality, impaired growth performance, reduced egg production, and poor egg quality [2, 3]. Transmission occurs through aerosolized respiratory secretions, direct contact with infected birds, and indirect exposure to contaminated feed, water, litter, equipment, and other fomites [4].

Co-infections with bacterial pathogens such as Escherichia coli, _Mycoplasma _gallisepticum, and _Mycoplasma _synoviae can aggravate the clinical severity of IB, increase mortality, and further complicate disease management [5]. Nephropathogenic IBV strains are of particular concern because they induce severe renal lesions that contribute to elevated mortality in broiler flocks [6]. Since its first identification in China in 2004, the QX variant of IBV has spread extensively throughout Asia, Europe, and other poultry-producing regions [7]. QX-like strains are associated with respiratory disease, nephropathogenic lesions, and reproductive disorders, particularly in young birds, resulting in long-term developmental abnormalities, including cystic oviducts and impaired reproductive performance [8–10].

In Thailand, several IBV genotypes, including TH1, TH2, and Massachusetts-type strains, have been identified [11]. The TH2 genotype is genetically closely related to the QX lineage and has been implicated in major disease outbreaks, posing a continuing challenge to effective disease control [12]. The extensive genetic diversity of IBV, driven primarily by mutations and recombination within the spike (S1) gene, facilitates immune evasion and reduces vaccine effectiveness [13]. Recombination among circulating IBV strains further promotes the emergence of novel serotypes that may escape immunity induced by existing vaccines [14]. Consequently, continuous molecular surveillance of circulating IBV strains is essential for developing and updating effective vaccination strategies.

Molecular epidemiological studies have demonstrated that QX-like IBV strains gradually replaced indigenous Thai IBV lineages after 2009 and have since become predominant in several regions of Thailand [10, 11]. These viruses are associated with respiratory disease, nephropathogenic lesions, impaired growth performance, and increased mortality in commercial broilers, resulting in considerable economic losses. Because both classical and variant IBV strains currently co-circulate in Thailand, vaccination strategies based on the protectotype concept have received increasing attention. This concept combines antigenically distinct vaccine strains to broaden heterologous protection against genetically diverse field viruses [15]. Accordingly, the combination of variant strain 1/96 and Massachusetts-type strain H120 has been proposed as a practical strategy to enhance tracheal protection against QX-like viruses. However, the effectiveness and potential limitations of concomitant administration of these vaccines under commercial broiler conditions remain insufficiently characterized.

Despite the widespread use of live-attenuated IBV vaccines, outbreaks caused by QX-like strains continue to occur in Thailand, indicating that currently implemented vaccination programs do not consistently provide adequate cross-protection against genetically diverse field viruses [16, 17]. Although previous investigations have evaluated single vaccine strains or sequential vaccination programs against heterologous IBV challenge, evidence regarding the simultaneous administration of two antigenically distinct live vaccines at day-old is limited, particularly in commercial broilers with maternally derived antibodies. Furthermore, most previous Thai studies have been conducted using specific-pathogen-free (SPF) chickens or vaccination schedules that differ from routine hatchery practices, thereby limiting the direct applicability of their findings to commercial production systems. Consequently, there remains a need to evaluate whether concomitant hatch-day vaccination with commercially available live-attenuated vaccines can provide effective cross-protection against currently circulating Thai QX-like IBV strains under conditions that closely resemble commercial broiler production.

Therefore, this study aimed to evaluate the protective efficacy of concomitant hatch-day vaccination with two commercially available live-attenuated IBV vaccines, Cevac IBird (strain 1/96; Ceva Animal Health, Libourne, France) and Cevac Vitabron L (strains H120 + PHY.LMV.42; Ceva Animal Health), against challenge with a Thai QX-like IBV strain in commercial broiler chickens. Protective efficacy was assessed by evaluating tracheal ciliary protection, viral detection by reverse transcription-polymerase chain reaction (RT-PCR), serological responses, clinical performance, and growth characteristics following challenge. We hypothesized that simultaneous exposure to two antigenically distinct live-attenuated vaccine strains at hatch would induce broader heterologous immunity and confer superior protection against Thai QX-like IBV infection compared with vaccination strategies based on a single vaccine strain.