The H5Nx viruses belonging to clade 2.3.4.4b has become enormous intimidation
globally. Outbreaks resulting from this clade have been reported in various countries
resulting in an exceptional numeral bird mortalities. This work aimed to assess 3 H5
vaccines of different clades (2.3.2 - 2.3.4 -2.3.4.4b) against newly emerged Egyptian clade
2.3.4.4bHPAI H5N1/2022 and H5N8/2019 viruses in commercial. broiler chickens. Clade
2.3.2 vaccine failed to protect clinically against both challenge viruses while, clade 2.3.4
(Re-5) showed 87 % and 40% protection against H5N8 and H5N1, respectively. Clade
2.3.4.4b showed 60% protection against H5N8 compared to 93% protection against
H5N1.Obtained results demonstrated that clade 2.3.4.4b H5N8 vaccine antigenically
matched effectively with the newly emerged clade 2.3.4.4b H5N1 virus& could afford high
level of clinical protection. Histopathological lesions were recorded in different collected
organs including cerebrum, trachea, lung, spleen, thymus and bursa of Fabricius. The
lymphoid tissues displayed moderate to severe lesion scores including congestion, necrosis
and depletion 0f lymphocytes. Thus, clade 2.3.4.4b viruses could have an
immunosuppressive effect. Continuous evaluation of the validated AI H5 vaccines against
newly emerging.AI strains is substaitional to deal with that intimidating remark. Also, the
vaccinal seed strains should updated regularly in order to obtain utmost protection levels
and assure presence of antigenic matching among prevailing strain and vaccinal strains.