Protective efficacy of prime-boost vaccination using rNDV-vectored IBV vaccine candidates in 1-day-old SPF chickens against IBV challenge. A Clinical signs in chickens immunized at day 1 and day 14 of age with the indicated vaccine candidates and challenged at day 35 of age with 105.2 EID50 per bird of Egyptian IBV variant strain EG/CU/4/2014. The birds were monitored daily for 10 days for clinical signs scoring (0 = no signs, 1 = mild ocular discharge, mild nasal discharge and/or sneezing 2 = heavy ocular discharge, heavy nasal discharge and/or coughing 3 = tracheal rales and/or mouth breathing). Mean score and SEM are plotted for each group. Statistically significant differences versus the control group at each time point are indicated by asterisk. B Determination of viral load (of IBV challenge virus) quantitative analysis of IBV RNA in tracheal and oropharyngeal swabs collected at 5 days post IBV challenge by RT-qPCR. Each point indicates the viral load for an individual chicken, the lines indicate the mean viral load for each group, and the error bars indicate SEM. Statistically significant differences are indicated by asterisk.