Continued use of IBV 793B vaccine needs reassessment after its withdrawal led to the genotype's disappearance

Over a period of almost two years, broilers chickens on several hundred Italian farms, were monitored for infectious bronchitis virus. Detections were genotyped using a hypervariable region of the gene coding for the S1 segment of the spike protein. A range of genotypes were detected which comprised QX, Q1, Mass, D274 and 793B. Sequences of 793B viruses detected in chickens, vaccinated with either of the two commonly used 793B type vaccines were almost identical to sequences of one or other of these vaccines. This strong indication of vaccine association led to the withdrawal of live 793B vaccine use on all of the farms of the study. Except for one sample collected soon after 793B vaccination ceased, it was no longer possible to detect 793B vaccine on these farms. It appears that field 793B strains have disappeared from the region of Italy tested thus obviating any need for current vaccine protection against 793B.