Effect of changes of vaccination strategies on IBV epidemiology, diagnosis and control: an Italian retrospective study.

Infectious bronchitis virus (IBV) is among the most impactful poultry pathogens, whose control, based on biosecurity and routine vaccination, is hampered by the existence of countless genetic variants sharing poor cross‑protection. A retrospective study was conducted on IBV positive samples collected in Italian broiler farms from 2012 to 2019. In 2015, the adopted vaccination protocol shifted from a Mass and 793B‑based vaccines to the administration of Mass and QX vaccines, allowing to study how changes in vaccination strategies may affect IBV epidemiology, control and diagnosis in the field. The most frequently detected lineages were QX (70.3%), 793B (15.8%) and Mass (11.9%). The relative frequencies of QX and 793B detections remained stable throughout the study, while Mass detections significantly increased after the vaccination change. Rather than to an actual growth of Mass population size, this finding may be attributable to different vaccine interactions, with Mass strains being more frequently concealed by 793B vaccines than by QX ones. Based on the obtained results, the two vaccination protocols appear to be similarly effective in fighting IB outbreaks, which in the last decade have been caused primarily by QX field strains in Italy. These results indicate that vaccination strategies may significantly affect IBV epidemiology and diagnosis, and should therefore be considered when choosing and interpreting diagnostic assays and planning control measures.