2.1  Vaccine and viruses
The H120 vaccine strain is currently a widely used vaccine strain, created from 120 serial passages of the H strain isolated in 1956. The ck/CH/IBYZ/2011 strain (referred to as IBYZ, GenBank KF663561.1) used in this study is classified as a QX-like IBV isolated from a flock presenting with IB symptoms by our research group in 2011 in Jiangsu Province, China. Both of these strains were recovered from a full-length clone using reverse genetics, as previously described [35], [36].