Different virulence of G4 and G5 Eurasian avian-like H1N1 swine influenza viruses in mice

Recently, a study reported a novel reassortant influenza A (H1N1) virus generated by the 2009 pandemic H1N1 (pdm/09 H1N1) viruses and swine viruses circulating in a swine herd in Shandong Province, eastern China (Yu et al., 2020). The influenza virus is an important pathogen in human, mammalian, and avian species. Pigs are recognized as a “mixing vessel” of human, swine, and avian influenza viruses, owing to their susceptibility to infection by these viruses. Swine influenza viruses (SIVs) also sporadically cause human infections (Li et al., 2019; Qi et al., 2013). At least 40% of the pig population worldwide was fed in China, where several SIV lineages have been reported in pigs, including classical swine H1N1, Eurasian avian-like (EA) H1N1, pdm/09 H1N1, triple reassortment influenza viruses (TRIG), and H3N2 (Ma, 2020). EA H1N1 SIVs were first identified in Europe in 1979. Since EA H1N1 SIVs were reported in Hong Kong in 2001, these viruses have gradually become the predominant subtype in pigs in China (Feng et al., 2021). Notably, several human cases of infection with EA H1N1 SIVs have been documented in particular countries, including the Netherlands and China (Li et al., 2019; Parys et al., 2021). Sun et al. (2020) demonstrated that EA H1N1 SIVs circulating in China could be clustered into six groups, G1-G6.