The highly pathogenic avian influenza (HPAI) H5N8 virus belonging to clade 2.3.4.4b
was detected in Egypt during 2016, which is one of the few countries depends on
vaccination as a main control strategy. This study was conducted to assess the efficacy of
three different commercial inactivated AI vaccines of different clades (clade 1, clade
2.3.4.4b and (Re-6 & Re-8)) in commercial broiler chickens against HPAI H5N8 clade
2.3.4.4b.The clinical protection percent was 67%, 80% and 87% in G1 (clade 1), G2 (clade
2.3.4.4b), and G3 (Re-6 & Re-8), respectively. The lowest mean antibody titer was recorded
in G1 which exhibited unsatisfactory level of immune response. Also, Antisera raised
against clade 1 vaccine showed minimal reactivity with clade 2.3.4.4b and Re-6&Re8antigens suggesting poor cross-protection. This was reflected on the antigenic relatedness
(R- value) which was low (11.3%) indicating major antigenic difference. Samples were
collected for examination of histopathological changes from thymus, trachea, spleen, bursa
of Fabricius, lung, and cerebrum. Only mild cerebrum lesions were found in all challenged
groups indicating reduced neurotropism. In lymphoid tissues, moderate to severe lesion
score were detected. This impairment effect on some bird immune organs highlight the
immunosuppressive effect of the virus, which can appear in the vaccinated exposed birds
to HPAI virus. It is recommended to update the vaccine seed strains to be closely related to
the circulating field strains to obtain better protection levels. Also, continuous evaluation of
the validated AI vaccines against recent field strains is substantial.