Cross-protection by inactivated H5 pre-pandemic vaccine seed strains against diverse Goose/Guangdong lineage H5N1 highly pathogenic avian influenza viruses.

The highly pathogenic avian influenza virus (HPAIV) H5N1 A/goose/Guangdong/1996 lineage (Gs/GD) is endemic in poultry across several countries in the world, and has caused lethal, sporadic infections in humans. Vaccines are important in HPAI control for both poultry and in pre-pandemic preparedness in humans. This study assessed inactivated pre-pandemic vaccine strains in a One Health framework, focusing on the genetic and antigenic diversity of field H5N1 Gs/GD viruses from the agricultural sector and assessing cross protection in a chicken challenge model. Nearly half (47.92%) of the forty-eight combinations of vaccine/challenge viruses examined had bird protection of 80% or above. Most vaccinated groups had prolonged mean death time (MDT) and the virus shedding titers were significantly lower compared to the sham group (p≤ 0.05). The antibody titers in the pre-challenge sera were not predictive of protection. Although vaccinated birds had higher titers of hemagglutination inhibiting (HI) antibodies against homologous vaccine antigen, most of them also had lower or no antibody titer against the challenge antigen. The comparison of all parameters, homologous or closely related vaccine and challenge viruses, gave the best prediction protection. Through additional analysis, we identified a pattern of epitopes substitutions in the hemagglutinin (HA) of each challenge virus that impacted protection, regardless of the vaccine used. These changes were situated in the antigenic sites and/or reported epitopes associated with virus escape from antibody neutralization. As a result, this study highlights virus diversity, immune response complexity, and the importance of strain selection for vaccine development to control H5N1 HPAIV in the agricultural sector and for human pre-pandemic preparedness. We suggest that the engineering of specific antigenic sites can improve the immunogenicity of H5 vaccines.ImportanceThe sustained circulation of highly pathogenic avian influenza virus (HPAIV) H5N1 A/goose/Guangdong/1996 lineage (Gs/GD) in the agricultural sector and some wild birds has led to the evolution and selection of distinct viral lineages involved in the escape from vaccine protection. Our results using inactivated vaccine candidates from the human pandemic preparedness program in a chicken challenge model identified critical antigenic conformational epitopes on the H5 hemagglutinin (HA) from different clades that were associated with antibody recognition and escape. Even though other investigators have reported epitope mapping in the H5 HA, much of this information pertains to epitopes reactive towards mouse antibodies. Our findings validate changes in antigenic epitopes of HA associated with virus escape from antibody neutralization in chickens, which has direct relevance to field protection and virus evolution. Therefore, the knowledge of these immunodominant regions is essential to proactively develop diagnostic tests, improve surveillance platforms to monitor AIV outbreaks, and design more efficient and broad-spectrum agricultural and human prepandemic vaccines.