Efficacy of rHVT-AI Vector Vaccine in Broilers with Passive Immunity Against Challenge with Two Antigenically Divergent Egyptian Clade 2.2.1 HPAI H5N1 Strains

In countries where avian influenza has become endemic, early vaccination of layer pullets or broilers with classical inactivated vaccines at the hatchery is no longer an option because of interference with passive immunity indirectly induced by the necessary vaccination of the breeders. On the other hand, injection of thousands of chicks from 7 to 10 days old on farms has been determined to be unreliable and, therefore, poorly efficacious. For these reasons, interest has arisen regarding a newly developed live recombinant vector vaccine based on a turkey herpesvirus (HVT) expressing the H5 gene from a clade 2.2 H5N1 highly pathogenic avian influenza virus (HPAIV) strain (rHVT-H5), which in theory is capable of breakthrough passive immunity to both the vector (HVT) and the insert (H5) and is consequently applicable at the hatchery. The objectives of this trial were to evaluate the impact of maternally derived antibodies (MDAs) specific to H5N1 on the immunity and the efficacy (protection and virus shedding) of different vaccination programs including rHVT-H5 and inactivated H5N1 and H5N2 vaccines applied alone or in combination. Therefore, broilers carrying MDAs against both HVT and Asian H5N1 HPAIV were vaccinated on the first day of age with rHVT-H5, with or without boosting vaccination by an inactivated vaccine after 10 days. The different groups were challenged with two antigenically highly divergent Egyptian clade 2.2.1 H5N1 HPAIVs at 4 wk of age. Protection against challenge was compared with unvaccinated birds or vaccinated birds without MDAs. Between 70% and 90% clinical protection could be observed in the vaccinated groups possessing MDAs, indicating no or very low interference of MDAs with vaccination. Results regarding clinical protection, humoral, cell-mediated, and mucosal immunity, as well as re-excretion of challenge virus are presented and discussed.