RESUMEN
Two HPAI H5N1 viruses were isolated from vaccinated layer and broiler commercial poultry farms in Egypt at years 2011 and 2013; respectively. By phylogenetic analysis, the viruses fall into two genetically diverse clades: [i] A/chicken/Egypt/VRLCU67/2011 classified as a variant virus, clade 2.2.1.1; and [ii] A/chicken/Egypt/13VIR3729-4/2013 classified as a classic virus, clade 2.2.1. Cross HI-test confirmed that the reaction between the two viruses is weak; furthermore, it showed the antigenic diversity between viruses belong to different clades and antigenic groups. Antigenic relatedness was calculated between six AI antigens and their antisera representing the different clades and antigenic groups circulated in Egyptian field; including the A/chicken/Egypt/VRLCU67/2011 strain which showed very low R-values with the other viruses' groups; ranging from 17 % to zero. Results demonstrated the genetic and antigenic diversity of the variant viruses and how can the vaccine seed be a weak point in the vaccination program that could be broken by the drifted viruses antigenically distant from the vaccine strain