The growth kinetics of the rIBVs were assessed by inoculation with 107 copies of virus per egg in 6 eggs from each group. The relative viral load was determined at 12, 18, 24, 36, 48, 72, and 96 h post-inoculation by quantitative real-time RT-PCR. The growth curves of all three strains reached a relatively stable plateau at 36–48 h after infection; the level of viral RNA for the rH120-S1/YZ strain was significantly higher than that of the rIBYZ strain for the majority of the time points (Fig. 2C).