Successive Efficacy Evaluation of Various Commercial Live-Attenuated Avian coronavirus Vaccination Schedules Against a Local GI-23.3 Challenge in SPF Broilers.

Background:Infectious bronchitis virus (IBV) is a highly spreading, evolving virus that induces multiple manifestations, including respiratory, urinary, and reproductive symptoms, and presents a considerable risk to the Egyptian poultry sector. This study assessed various IBV vaccination protocols available in broiler populations comprising live attenuated vaccines such as IB Var II, 793/B (4/91), IB Primer, and H120 against the local novel IBV-GI-23.3 strain. Methods: Vaccines were administered to eight groups of SPF chicks at 1 day only or 1 + 14 days of age. Birds were challenged via the oculo-nasal route at 28 days of age using 10⁶ EID₅₀/0.2 mL/chick with the NewValley-1-EGYIBV-GI23.3-2023 local strain. Ciliostasis activity and the scores for histopathological lesions were evaluated at 7 days post-challenge (DPC). Virus shedding was monitored at 3, 5, and 7 DPC using the real-time RT-PCR method. Results: The ciliostasis test indicated that the vaccinated groups receiving the IB Primer + 4/91 vaccine regime at 1 day only or 1 + 14 days of age received the highest level of protection (65%, 68%, respectively). Similarly, administration of IB Primer + IB Var II at 1 + 14 days of age demonstrated substantial protection (63%). Conversely, administering the H120 + 4/91 vaccination protocol at days 1 and 14 resulted in a moderate level of protection (53%). Tracheal IBV shedding quantification and subsequent histopathological signs of trachea, proventriculus, bursa, and kidney degenerative changes were significantly lower in the vaccinated groups (especially the IB Primer + 4/91 vaccine regime at 1 day only or 1 + 14 days) than in the positive control groups. Conclusions: The heterologous combined IB Primer + 4/91 program demonstrated the most significant protective efficacy against the IBV field challenge strains compared with other vaccines in broiler chickens.