Recombinant HA1-ΔfliC enhances adherence to respiratory epithelial cells and promotes the superiorly protective immune responses against H9N2 influenza virus in chickens

•The HA1-ΔfliC protein is recognized by TLR5 and induces a moderate innate immune response compared to HA1-fliC and fliC-HA1.•The HA1-ΔfliC protein exhibits a higher adherence to the nasal cavity and trachea than HA1-fliC and fliC-HA1.•The HA1-ΔfliC protein significantly increases the HA-specific sIgA titers than inactivated vaccine.•Chickens treated with HA1-ΔfliC have a significantly reduced level of viral loads in the cloaca and throat compared with chickens treated with inactivated vaccine.