The Efficacy of Inactivated Vaccine Against H5 Clade 2.3.4.4b Highly Pathogenic Avian Influenza Virus in Turkeys.

The global outbreak of clade 2.3.4.4b H5 highly pathogenic avian influenza (HPAI) has caused tremendous losses in poultry worldwide. Although turkeys are a smaller sector in poultry production compared to chickens, they tend to be affected more severely by HPAI because they can usually be infected with a lower dose of virus (i.e., they are more susceptible). Vaccines can be effective tools to help control HPAI, but data on vaccine efficacy and antibody response in turkeys are somewhat limited. Here we evaluated an in-house-produced, reverse-genetics-generated H5N9 inactivated vaccine that has a clade 2.3.4.4b H5 HA from A/turkey/Indiana/22-003707-003/2022 (TK/IN/22) modified to be low pathogenic and a North American wild bird lineage N9 in a PR8 "backbone" for its efficacy in commercial broad-breasted white turkeys by homologous challenge. Turkeys were divided into three vaccination groups, where each group was vaccinated once at 3, 7, or 9 wk of age. Turkeys were challenged at 10 wk of age with TK/IN/22 HPAI virus. There was 100% survival in all vaccinated groups and 0% survival in the sham immunized group. A significant decrease in viral shedding was observed in all vaccinated groups compared to the sham immunized turkeys. Also, the 9 wk vaccination group shed significantly higher quantities by the cloacal route at 7 days postchallenge (DPC) compared to the 3 wk vaccination group, and two turkeys in the 9 wk vaccination group had mild clinical signs 6-7 DPC. The neuraminidase inhibition-enzyme-linked lectin assay (NI-ELLA) was used to evaluate antibodies to the vaccine and was more sensitive than the hemagglutinin inhibition assay. Also, when tested as a potential assay to differentiate vaccinated and infected animals, 50% to 90% of vaccinated turkeys (depending on the age at vaccination) were positive by NI-ELLA at 7 DPC for antibodies to the challenge virus, and 100% were positive at 14 DPC.