Highly pathogenic avian influenza virus H5N1 from Egypt escapes vaccine-induced immunity but confers clinical protection against a heterologous clade 2.2.1 Egyptian isolate.

The poultry populations of Egypt are endemically infected by highly pathogenic avian influenza viruses (HPAIV) of subtype H5N1. Vaccination was chosen as an auxiliary tool to control HPAIV in poultry. Potency of commercial vaccines regarding emerging variants is under discussion. In the current study efficacy of four different inactivated whole H5 virus vaccines representing different sublineages of HPAIV H5N1 were tested in chickens against challenge viruses currently co-circulating in Egypt and representing two antigenically widely distinct HPAIV H5N1 lineages, i.e., "variant" (clade 2.2.1var) and "proper" (clade 2.2.1pro) viruses. All vaccines induced clinical protection against challenge with 2.2.1pro Egyptian strains. In contrast, when challenged with a variant strain, only chickens vaccinated with the homologous Egyptian clade 2.2.1var virus or an inactivated re-assorted H5N1 strain (Re-5, clade 2.3) were protected. However, only the homologous virus induced sterile immunity whereas chickens clinically protected after Re-5 vaccination shed virus at day two after infection indistinguishable to H5N2 vaccines. In conclusion, monitoring vaccine-driven evolution of HPAIV H5N1 by surveillance, antigenic characterization, and challenge studies is essential to assess efficacy of AIV vaccination campaigns.