2.8.1  Viral challenge
In total, 30 one-day-old SPF chickens were divided into three groups of 10 chickens. The three groups were intranasally vaccinated with the rH120 and rH120-S1/YZ strains at a dose of 104 EID50/200 μL, and the control group received PBS. Two weeks post-vaccination, the chickens in each group were challenged with the QX-like strain, rIBYZ, at 106 EID50/200 μL via eye drop and the intranasal route. Any clinical signs, as well as the percentage of morbidity and mortality were recorded for 14 days. The dead chickens were also examined for gross tracheal, lung, and kidney lesions.