Development of a Novel Live Attenuated QX-Like Infectious Bronchitis Virus Vaccine and Its Efficacy Against Recent GI-19 Subgroup Variants.

The emergence of novel nephropathogenic QX-like infectious bronchitis virus (IBV) strains is a major problem across multiple continents. Despite extensive vaccination efforts, the emergence of novel QX-like variant strains that lack serologic cross-reactivity with commercially available vaccines complicates the control of IBV. In the present study, a live attenuated IBV vaccine strain was developed by serial passages with heat adaptation on the basis of the currently circulating IBV strain K29/18 of the QX-like III subgroup, and its safety, immunogenicity, and cross-protective efficacy against KM91-like, QX-like III, and novel QX-like variant IBVs were evaluated. Unlike the parental strain, the attenuated strain, designated as K29/18 HP70, was found to be safe in 1-day-old specific-pathogen-free chicks, which showed neither mortality nor signs of morbidity after intraocular, spray, or oral administration. It induced high titers of neutralizing antibodies and provided significant protection to the respiratory tract and kidney against infection with the QX-like III and novel QX-like variant viruses. We conclude that the heat-adapted K29/18 HP70 vaccine strain exhibited the desired levels of attenuation, immunogenicity, and cross-protective efficacy required for use as a live vaccine in poultry hatchery spray and mass applications. The K29/18 HP70 strain developed in this study is a promising vaccine candidate for preventing and controlling QX-like IBV infections.