In Russia, during the COVID-19 pandemic, a decrease in influenza circulation was initially observed. Influenza circulation re-emerged with the dominance of new clades of A(H3N2) viruses in 2021–2022 and A(H1N1)pdm09 viruses in 2022–2023. In this study, we aimed to characterize influenza viruses during the 2022–2023 season in Russia, as well as investigate A(H1N1)pdm09 HA-D222G/N polymorphism associated with increased disease severity. PCR testing of 780 clinical specimens showed 72.2% of them to be positive for A(H1N1)pdm09, 2.8% for A(H3N2), and 25% for influenza B viruses. The majority of A(H1N1)pdm09 viruses analyzed belonged to the newly emerged 6B.1A.5a.2a clade. The intra-sample predominance of HA-D222G/N virus variants was observed in 29% of the specimens from A(H1N1)pdm09 fatal cases. The D222N polymorphic variant was registered more frequently than D222G. All the B/Victoria viruses analyzed belonged to the V1A.3a.2 clade. Several identified A(H3N2) viruses belonged to one of the four subclades (2a.1b, 2a.3a.1, 2a.3b, 2b) within the 3C.2a1b.2a.2 group. The majority of antigenically characterized viruses bore similarities to the corresponding 2022–2023 NH vaccine strains. Only one influenza A(H1N1)pdm09 virus showed reduced inhibition by neuraminidase inhibitors. None of the influenza viruses analyzed had genetic markers of reduced susceptibility to baloxavir.

The circulation of seasonal influenza in 2020–2021 around the world was drastically reduced after the start of the COVID-19 pandemic and the implementation of mitigation strategies. The influenza virus circulation reemerged in 2021–2022 with the global spread of the new genetic clade 3C.2a1b.2a.2 of A(H3N2) viruses. The purpose of this study was to characterize influenza viruses in the 2021–2022 season in Russia and to analyze the receptor specificity properties of the 3C.2a1b.2a.2 A(H3N2) viruses. Clinical influenza samples were collected at the local Sanitary-and-Epidemiological Centers of Rospotrebnadzor. Whole genome sequencing was performed using NGS. The receptor specificity of hemagglutinin was evaluated using molecular modeling and bio-layer interferometry. Clinical samples from 854 cases of influenza A and B were studied; A(H3N2) viruses were in the majority of the samples. All genetically studied A(H3N2) viruses belonged to the new genetic clade 3C.2a1b.2a.2. Molecular modeling analysis suggested a higher affinity of hemagglutinin of 3C.2a1b.2a.2. A(H3N2) viruses to the α2,6 human receptor. In vitro analysis using a trisaccharide 6’-Sialyl-N-acetyllactosamine receptor analog did not resolve the differences in the receptor specificity of 3C.2a1b.2a.2 clade viruses from viruses belonging to the 3C.2a1b.2a.1 clade. Further investigation of the A(H3N2) viruses is required for the evaluation of their possible adaptive advantages. Constant monitoring and characterization of influenza are critical for epidemiological analysis.

This paper describes the current situation on highly pathogenic avian influenza virus in 2022 and provides forecast of the possible further spread of avian influenza in Russia. In 2022, the circulation of a wide variety of highly pathogenic avian influenza virus subtypes, which have epizootiological and epidemiological significance, was recorded in the world. Outbreaks caused by highly pathogenic avian influenza virus were reported in over 60 countries. In addition, human infections with influenza viruses of the A(H5Nx) and A(H9N2) subtypes were registered. There was a large-scale epizootic which affected more than 10 regions of the European part of Russia and the Russian Far East in 2022. Outbreaks among wild birds and poultry were caused by the highly pathogenic influenza virus A(H5N1) of the clade 2.3.4.4b, at the same time genetic and antigenic diversity was observed among viruses circulating in Russia. Thus, an essential geographical role of the territory of Russia in the global spread of avian influenza virus has been shown once again, which highlights the importance of continuous avian influenza virus surveillance in the country.

The WHO has developed a tool to assess the risk of newly emerging influenza viruses with pandemic potential (TIPRA). According to TIPRA, the main parameters for assessing the risk of human-to-human transmission of a novel influenza virus are its ability to bind to human cell receptors of the upper respiratory tract (URT) and transmit in model animals. The aim of this study was to quantify airborne transmission of human and animal influenza viruses in the ferret model. The transmission of influenza viruses was studied in the ferret model in an aerobiology chamber. Airborne particles concentration and fractional composition in the aerobiology chamber were measured using an aerosol particle counter and analytical aerosol filters. Viral load in ferret nasal washings and aerosol filters was determined by titration in MDCK cells and quantitative RT-PCR. Genetic analysis of influenza viruses was performed using virus genome sequences obtained by NGS. After intranasal infection, human and animal influenza viruses replicated in the cells of nasal mucosa in ferrets. The level of virus airborne particles contamination provided by infected animals depends on the infectious dose and differs significantly between influenza virus strains. The studied avian influenza viruses show insufficient transmission in the ferret model, while human and swine influenza viruses are highly transmitted in ferrets. We propose a quantitative model of airborne transmission of influenza virus from donor to recipient ferrets. Level of influenza virus transmission in the ferret model correlates with genetic markers of virus receptor specificity and the level of virus airborne particle contamination induced by donor ferrets.