Spread of H5N1 Avian Influenza Viruses Clade 2.3.4.4b in the Russian Federation, 2021–2023

A highly virulent avian influenza virus H5N1 clade 2.3.4.4b, which emerged in Europe in 2020 as a result of reassortment of the H5N8 virus, quickly began to spread through wild birds. By 2021, this virus had already been detected in the Russian Federation. Our whole-genome sequencing and phylogenetic analysis of isolates collected in Russia revealed several instances of virus introduction from European countries. In the period from 2021 to 2023, avian influenza viruses of genotypes EA-2020-C, EA-2021-AB, EA-2022-BB, EA-2023-DA and several ungenotyped reassortant isolates were detected in the Russian Federation. The territories of the Black Sea–Caspian Sea coast and the Russian Far East are noted as the main geographic locations that may serve as centers for the detection of new reassortant forms of viruses. Several key amino acid markers have been identified that indicate an increase in the zoonotic potential of avian influenza viruses; however, no multiple instances of avian influenza virus detection in mammals were documented in the Russian Federation between 2021 and 2023.