Global Emergence of Infectious Bronchitis Virus Variants: Evolution, Immunity, and Vaccination Challenges.

Infectious bronchitis is an acute, extremely contagious viral disease affecting chickens of all ages, leading to devastating economic losses in the poultry industry worldwide. Affected chickens show respiratory distress and/or nephritis, in addition to decrease of egg production and quality in layers. The avian coronavirus, infectious bronchitis virus (IBV), is a rapidly evolving virus due to the high frequency of mutations and recombination events that are common in coronaviruses. This leads to the continual emergence of novel genotypes that show variable or poor crossprotection. The immune response against IBV is complex. Passive, innate and adaptive humoral and cellular immunity play distinct roles in protection against IBV. Despite intensive vaccination using the currently available live-attenuated and inactivated IBV vaccines, IBV continues to circulate, evolve, and trigger outbreaks worldwide, indicating the urgent need to update the current vaccines to control the emerging variants. Different approaches for preparation of IBV vaccines, including DNA, subunit, peptides, virus-like particles, vectored and recombinant vaccines, have been tested in many studies to combat the disease. This review focuses on several key aspects related to IBV, including its clinical significance, the functional structure of the virus, the factors that contribute to its evolution and diversity, the types of immune responses against IBV, and the characteristics of both current and emerging IBV vaccines. The goal is to provide a comprehensive understanding of IBV and explore the emergence of variants, their dissemination around the world, and the challenges to define the efficient vaccination strategies.