In the past century, we have seen four pandemics caused by human influenza A: H1N1 Spanish influenza (1918–1920), H2N2 Asian influenza (1957–58), H3N2 Hong Kong influenza (1968–69), and H1N1 swine influenza (2009–10). Phylogenetic analyses have shown that reassortment of genetic segments from human, animal, or avian influenza viruses is linked to the origin of the four influenza pandemic strains.
Since 2013, there have been many novel avian influenza viruses that were identified from humans, including H7N9, H6N1, H10N8, H5N6, H7N4, H10N3, and H5N8 avian influenza viruses. Notably, H7N9 viruses have caused more than 1500 laboratory-confirmed human cases of avian influenza worldwide. In April, 2022, a 4-year-old boy with influenza-like illness was determined to have been infected with a novel H3N8 avian influenza virus in Henan province. Subsequently, H3N8 avian influenza virus caused a second human infection in Hunan province, in May, 2022. The continuous emergence of novel avian influenza viruses capable of human infection highlights the increasing pandemic risk that they pose.