Viral load in the different organs at 1, 3, 5, and 7 dpi of SPF chickens infected with the recombinant strains rH120, rIBYZ, and rH120-S1/YZ at 1-day-of age. The bars indicate means ± standard deviations. ***Indicates an extremely significant difference at P ≤ 0.001; **Indicates a highly significant difference at 0.001 < P ≤ 0.01; *Indicates a significant difference at 0.01 < P ≤ 0.05.