Long-Term Immunogenicity and Protection of a rHVT-H9/Y280 Vaccine Against H9N2 Avian Influenza Virus in Commercial Layers with High Maternal Antibodies

The endemicity of H9N2 avian influenza viruses (AIVs), particularly the Y280 lineage, poses persistent challenges to the poultry industry due to the limitations of inactivated vaccines, such as interference by maternally derived antibodies (MDAs) and incomplete suppression of viral replication. This study evaluated the immunogenicity and protective efficacy of a novel recombinant turkey herpesvirus vaccine expressing the hemagglutinin gene of H9N2/Y280 (rHVT-H9/Y280) in commercial Hy-Line Brown layers with high-MDA backgrounds. In a comparative challenge study, the rHVT-H9/Y280 vaccine induced complete protection against a homologous Y280 strain challenge at 4 weeks of age, whereas commercial inactivated vaccines failed to completely block replication, showing virus isolation rates of 16.7–25%. Long-term serological monitoring demonstrated that the rHVT-H9/Y280 vaccine elicited a robust humoral response characterized by persistent maintenance of high HI titers (>8.0 log2) up to 39 weeks post-vaccination. These findings confirm that rHVT-H9/Y280 effectively overcomes MDA interference and provides protection by inhibition of viral replication in layer chickens, making it a promising candidate for the effective control of H9N2 AIV in endemic regions.