Vaccine Efficacy of Inactivated, Chimeric Hemagglutinin H9/H5N2 Avian Influenza Virus and Its Suitability for the Marker Vaccine Strategy

In order to produce a dually effective vaccine against H9 and H5 avian influenza viruses that aligns with the DIVA (differentiating infected from vaccinated animals) strategy, we generated a chimeric H9/H5N2 recombinant vaccine that expressed the whole HA1 region of A/CK/Korea/04163/04 (H9N2) and the HA2 region of recent highly pathogenic avian influenza (HPAI) A/MD/Korea/W452/14 (H5N8) viruses. The chimeric H9/H5N2 virus showed in vitro and in vivo growth properties and virulence that were similar to those of the low-pathogenic avian influenza (LPAI) H9 virus. An inactivated vaccine based on this chimeric virus induced serum neutralizing (SN) antibodies against both H9 and H5 viruses but induced cross-reactive hemagglutination inhibition (HI) antibody only against H9 viruses. Thus, this suggests its compatibility for use in the DIVA strategy against H5 strains. Furthermore, the chimeric H9/H5N2 recombinant vaccine protected immunized chickens against lethal challenge by HPAI H5N8 viruses and significantly attenuated virus shedding after infection by both H9N2 and HPAI H5N8 viruses. In mice, serological analyses confirmed that HA1- and HA2 stalk-specific antibody responses were induced by vaccination and that the DIVA principle could be employed through the use of an HI assay against H5 viruses. Furthermore, each HA1- and HA2 stalk-specific antibody response was sufficient to inhibit viral replication and protect the chimeric virus-immunized mice from lethal challenge with both mouse-adapted H9N2 and wild-type HPAI H5N1 viruses, although differences in vaccine efficacy against a homologous H9 virus (HA1 head domain immune-mediated protection) and a heterosubtypic H5 virus (HA2 stalk domain immune-mediated protection) were observed. Taken together, these results demonstrate that the novel chimeric H9/H5N2 recombinant virus is a low-pathogenic virus, and this chimeric vaccine is suitable for a DIVA vaccine with broad-spectrum neutralizing antibody against H5 avian influenza viruses.IMPORTANCE Current influenza virus killed vaccines predominantly induce antihemagglutinin (anti-HA) antibodies that are commonly strain specific in that the antibodies have potent neutralizing activity against homologous strains but do not cross-react with HAs of other influenza virus subtypes. In contrast, the HA2 stalk domain is relatively well conserved among subtypes, and recently, broadly neutralizing antibodies against this domain have been isolated. Therefore, in light of the need for a vaccine strain that applies the DIVA strategy utilizing an HI assay and induces broad cross-protection against H5N1 and H9N2 viruses, we generated a novel chimeric H9/H5N1 virus that expresses the entire HA1 portion from the H9N2 virus and the HA2 region of the heterosubtypic H5N8 virus. The chimeric H9/H5N2 recombinant vaccine protected immunized hosts against lethal challenge with H9N2 and HPAI H5N1 viruses with significantly attenuated virus shedding in immunized hosts. Therefore, this chimeric vaccine is suitable as a DIVA vaccine against H5 avian influenza viruses.