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Abstract Currently A(H5Nx) avian influenza viruses are globally widespread and continue to evolve. Since their emergence in 2020 novel highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b reassortant viruses have become predominant in the world and caused multiple infections in mammals. It was shown that some of A(H5N1) viruses mostly isolated from mammals contain an E627K mutation in the PB2 protein which can lead to adaptation of influenza viruses to mammalian cells. In 2023 in Russia we have isolated two highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b viruses from birds one of which contained an E627K mutation in the PB2 protein. This virus had increased virulence in mice. Limited airborne transmission of the virus with the PB2-E627K mutation was observed between ferrets, in which infectious virus was detected in the nasal washings of the three of the twelve recipient ferrets, and clinical symptoms of the disease were observed in one case. Both viruses showed dominant binding to avian-type sialoside receptors, which was most likely the reason for the limited transmissibility. Thus, this study indicates a possible limited increase in the pandemic potential of A(H5N1) 2.3.4.4b viruses and highlights the importance of continuous avian influenza surveillance for pandemic preparedness and response.