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Introduction
Infectious bronchitis virus (IBV) is a Gammacoronavirus that causes a highly contagious disease in chickens. The virus causes severe economic losses to the poultry industry worldwide because it can affect the upper respiratory and reproductive tracts, and some strains can cause nephritis in chickens [1] . It is well known that the primary problem in the control of infectious bronchitis is the ability of the virus to generate antigenic diversity by inaccuracy of the coronavirus RNA-dependent RNA polymerase and high frequency of homologous RNA recombination [2,3] . Many studies have shown that the degree of cross-protection tends to decrease among IBV serotypes and genotypes [4,5] .
Despite intensive vaccination efforts using attenuated live and killed vaccines to prevent the disease, the emergence of new variant strains that do not serologically cross-react complicates disease control and is an argument for vaccinating chickens with the type of IBV causing the disease [3,6] . However, producing a live IBV vaccine requires lengthy strategies that are time, cost, and labor intensive. Attenuation of IBV by multiple passages (over 100) in embryonated eggs can delay a new vaccine's clinical availability for several years. This drawback is compounded by the time required for verification of the vaccine to obtain licensing, as well as the current lack of available cell lines for vaccine production, which could accelerate vaccine production. Furthermore, there is no guarantee that the viral strain used to produce the vaccine will still be endemic at the time of vaccination [7-9] .
Our previous study revealed that the QXIBV strain originating from China has been introduced in Korea and has formed a new cluster in the field [10] . This new cluster, represented by the K40/09 strain, is a natural recombinant strain between the Korean nephropathogenic strain KM91 and the QXIBV strain. In a previous challenge study, we characterized the Korean variant IBV K40/09 strain with regard to its immunogenicity and cross-protective efficacy against heterotype strains and its potential as a vaccine candidate [11] .
The development of a temperature-adapted vaccine has been attempted to increase safety and shorten the time of attenuation in several studies [12-14] . However, our experience shows that, besides desirable levels of attenuation and immunogenicity, other traits such as good growth properties should...