The H1N1 influenza virus of swine-origin was responsible for the H1N1 pandemic in 2009 (pdm/09 H1N1), where the virus was transmitted to humans and then spread between people, and its continued circulation has resulted in it becoming a seasonal human flu virus. Since 2016, the matrix (M) gene of pdm/09 H1N1 has been involved in the reassortment of swine influenza viruses (SIVs) in China and has gradually become a dominant genotype in pigs. However, whether M gene substitution will influence the fitness of emerging SIVs remains unclear. Here, we analyzed the biological characteristics of SIVs with the M gene from Eurasian avian-like (EA) SIV or pdm/09 H1N1 in mammals and found that SIVs containing the pdm/09-M gene exhibit stronger virulence in mice, more efficient respiratory droplet transmission between ferrets, and increased transcription of viral genes in A549 cells compared with those containing EA-M. We also determined the functional significance of...

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