Seroprevalence of anti-influenza antibodies in humans and characterization of seasonal influenza viruses isolated in Russia during the 2023–2024 flu season

The 2023–2024 flu season in Russia was dominated by influenza A(H3N2) viruses. In this study, we isolated seasonal influenza viruses from human respiratory specimens, analyzed their genetic and antigenic properties, and assessed their susceptibility to neuraminidase inhibitors. In total, we isolated 207 influenza virus isolates. Of them, 95.2% were subtyped as A(H3N2), 1.9% as A(H1N1)pdm09, and 2.9% as B/Victoria influenza viruses. The hemagglutinin sequences of the A(H3N2) isolates showed that they belonged to several subclades within the 2a.3a.1 genetic group, with the J2 subclade being predominant. Despite their genetic diversity, all A(H3N2) strains tested using a hemagglutination inhibition assay were antigenically similar to the egg-propagated vaccine strain A/Darwin/09/2021(H3N2). The B/Victoria virus isolates belonged to the C.5 and C.5.7 subclades of the genetic group V1A.3a.2 and were antigenically similar to cell- and egg-propagated variants of the vaccine strain B/Austria/1359417/2021. All of the A(H1N1)pdm09 isolates belonged to the 6B.1A.5a.2a genetic clade and were well-recognized by a ferret antiserum raised against a cell-propagated A/Wisconsin/67/2022(H1N1pdm09)-like reference strain. All of the tested isolates were susceptible to oseltamivir and zanamivir, including two A(H1N1)pdm09 strains with an NA-S247N substitution. Seroprevalence analysis showed that 60%, 54%, and 46% of the human blood samples tested were seropositive for the A(H3N2), A(H1N1)pdm09 and B/Victoria antigens from the 2022–2023 vaccine.