Investigation into the efficacy of a commercially available inactivated avian influenza virus (AIV) vaccine (H9N2) through experimental and field assessments, with an emphasis on its compatibility with recent AIV (H9N2) G1-sublineage isolates.

Avian influenza H9N2 virus has established itself as endemic in various parts of the globe notably in the Middle East with significant economic repercussions. Vaccination with inactivated vaccine is regarded as one of the most effective strategies for disease prevention. To enhance the efficacy of the vaccination protocol the seed strain must possess genetic and antigenic characteristics closely resemble to the circulating field viruses. This research sought to assess the effectiveness of an oil-inactivated avian influenza virus (AIV) H9N2 vaccine given at different dosages and ages in broiler chicks as well as in commercial layer chicken flocks. This assessment involved challenging the chicks with the H9N2 G1 lineage avian influenza virus (A/chicken/Egypt/v1896/2022 (H9N2)) at 14 and 21 days of age alongside conducting serological monitoring in a layer flock. Phylogenetic analysis of the vaccine seed strains revealed to more than 99.00% amino acid identity to the recently isolated strains. All applied vaccination protocols successfully reduced virus shedding, clinical symptoms, and pathological damages in the trachea and lung. Birds that received a full dose of the inactivated H9N2 vaccine at zero and three days of age exhibited the strongest immune responses by 28 days even in the presence of maternal-derived antibodies. Furthermore, administering the inactivated H9N2 vaccine at three days of age provided the most substantial clinical protection against challenges occurring at various time points. Implementing a priming-boost vaccination strategy for layers provided the birds with adequate humoral immune responses to achieve sufficient pre-laying titres.