Molecular surveillance of GI-19 lineage infectious bronchitis virus in China: divergent evolution and efficacy assessment of QXL87 vaccine against heterologous clades

BACKGROUND: Infectious bronchitis is a highly contagious acute disease in chickens caused by the infectious bronchitis virus (IBV). The GI-19 lineage of IBV has been shown to undergo continuous evolution, with increasing variance in antigenicity and pathogenicity in China. The aim of this study was to assess the efficacy of the only registered QX vaccine (i.e., QXL87) in China against diverse GI-19 clades. RESULTS: A total of 1503 S1 sequences from China were identified as belonging to the IBV GI-19 based on phylogenetic analysis, including sequences from GenBank and clinical samples sequenced by our team (Key Laboratory of Animal Infectious Diseases of Ministry of Agriculture, China) from 2018 to 2022. Within the GI-19 lineage, five clades were identified and QXL87 vaccine belonged to clade 3. Clade 2 and clade 4 were found to be prevalent from 2018 to 2022. One strain was randomly selected from each of the four clades distinct from the QXL87 vaccine strain to serve as challenge strains for testing the cross-protective efficacy of the QXL87 vaccine. The amino acid homology of the S1 protein between these strains and the QXL87 vaccine strain ranged from 93.7 to 94.8%. The QXL87 vaccine was demonstrated clinical protection rates of 90%, 80%, 90%, and 90% against four respective strains. It exhibited protection rates above 70% for ciliary activity against all strains tested (74.29% for the GD4 strain, 79.33% for the 06Ⅱ strain, 81.63% for the JS5 strain, and 90.31% for the JS41 strain). Vaccinated chickens experienced milder clinical signs, fewer lesions and reduced viral shedding in the trachea and kidneys. CONCLUSIONS: This study underscored the substantial genetic divergence within the dominant GI-19 lineage of IBV, resulting in the emergence of multiple clades. Despite this, the QXL87 vaccine remained effective for preventing GI-19 lineage IBV infection although the protection rates differed among different clades. However, we must acknowledge that in cross-protection trials, ciliary motility scores for clade 1 and clade 3 marginally dropped below 80% according to a more accurate calculation method published. Consequently, ongoing surveillance for mutations of circulating IBV strains remains critical. In case the antigenic drift compromises vaccine efficacy, timely updating vaccine strains or immunization strategies involving multiple vaccine candidates should be prioritized to ensure sustained protection.