This study aimed to characterize Egyptian H5N8 viruses that were recovered from vaccinated commercial layer flocks in the Al-Sharqia governorate in Egypt during 2019 and 2021. Due to respiratory problems and a sudden, sharp drop in egg production, ten of the fifteen vaccinated farms were assessed for this study, with a fatality rate ranging from 15% to 50%. Using qualitative reverse transcriptase-polymerase chain reaction (RT‒qPCR) and virus isolation from embryonated chicken eggs (ECEs) for first- and second-blind passages, avian influenza virus (AIV) testing was positive in ten of the fifteen flocks that were inspected. For isolates that passed the hemagglutination test, the hemagglutinin (HA) and neuraminidase (NA) genes were partially amplified using qualitative PCR. Five isolates that were representative of five multilayer flocks were then subjected to sequencing and phylogenetic analysis. Our findings revealed the AIV/HPAI H5N8 virus by sequencing and phylogenetic analysis of the HA and NA genomes of five flocks. The four HAs and NAs shared 99%. 99.8% and 98.1% of their nucleotide sequences with H5N8 viruses that were previously discovered in China, Iran, and Iraq, respectively. Furthermore, phylogenetic analyses focusing on the HA and NA genes revealed that Egyptian H5N8 viruses belonged to group B, with Russian-like reassorted H5N8 viruses in clade 2.3.4.4. In conclusion, birds that have received immunizations are at serious risk from HPAI H5N8. Detection of the HPAI H5N8 virus in domestic birds is therefore required for yearly surveillance, sporadic molecular monitoring, and evaluation of the effectiveness of immunization.