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

## TL;DR

A new vaccine for H9N2 avian influenza in chickens works even with maternal antibodies and provides long-term protection.

## Contribution

A novel recombinant turkey herpesvirus vaccine (rHVT-H9/Y280) overcomes maternal antibody interference and provides long-term protection against H9N2 avian influenza in commercial layers.

## Key findings

- The rHVT-H9/Y280 vaccine provided 100% protection against H9N2 virus and blocked viral replication in internal organs.
- The vaccine induced long-lasting immunity with high antibody levels persisting for up to 39 weeks.
- Unlike traditional vaccines, it was effective in chickens with high maternal antibodies.

## Abstract

H9N2-subtype avian influenza (H9N2) is a widespread endemic disease causing significant economic losses in the global poultry industry. Currently, control relies mainly on inactivated vaccines, but their efficacy is often limited by interference from maternally derived antibodies (MDAs) in chicks and an inability to completely prevent virus spread. This study evaluated a new-generation vaccine, rHVT-H9/Y280, which uses a turkey herpesvirus vector to deliver H9N2 protection. We tested this vaccine in commercial layer chickens with high levels of MDA. The results showed that, unlike traditional vaccines, the rHVT-H9/Y280 vaccine was not affected by MDAs and provided 100% protection against the virus, completely blocking viral replication in internal organs. Furthermore, a single dose provided long-lasting immunity, with antibody levels persisting for up to 39 weeks. These findings suggest that this novel vaccine can effectively prevent infection and transmission even in young chicks with maternal immunity, helping to purify poultry flocks.

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.