“…It has been reported that the side effects of the 4/91 vaccine might provide the basis for recombination with other strains. This phenomenon has been shown by several studies which have traced the emergence of many novel IBV strains to recombination events between the 4/91 vaccine and field strains (Feng et al, 2018;Han et al, 2017;Jiang et al, 2017Jiang et al, , 2018Liu et al, 2013;Mo et al, 2013;Ovchinnikova et al, 2011;Zhang et al, 2015Zhang et al, , 2016Zhou et al, 2017) in cases where the 4/91 vaccine strain could not provide effective protection against the field strains , leading to coinfection of the vaccine and field strains in the same chickens and ultimately resulting in recombination. In this study, we performed complete genome sequence analysis of strains I0718/17, I0722/17, I0724/17, and I0737/17 and confirmed the occurrence of recombination events at the 5′ ends of the S1 gene between a 4/91-like virus (GI-13 lineage) and a YX10-like virus (GI-19) (Xu et al, 2018), which contributed to the emergence of the IBV strains.…”