Scientists propose alternate origins for 1918 pandemic flu virus
The 1918 H1N1 pandemic influenza virus may have originated from reassortment between a human flu virus lineage and an avian flu virus, researchers speculate today after conducting a phylogenetic evolutionary analysis. The findings run contrary to previous estimates of the virus’s evolution.
US and UK researchers analyzed full-length gene sequences from human, bird, and swine viruses over the years involving H1, H2, H5, and N1 subtypes, according to their study in Proceedings of the National Academy of Sciences.
Their results indicate that human H1 (the “H” is for the hemagglutinin [HA] protein) emerged from an avian source well before 1918 but after 1895. They also noted that the classic swine flu lineage is nested completely within the 1918 genetic diversity of human H1 but the seasonal human H1 HA line is only distantly related to the pandemic 1918 HA.
“This pattern indicates that the swine influenza lineage emerged directly from the human pandemic virus but that postpandemic seasonal H1N1 did not,” they write. “Rather, there is strong phylogenetic evidence that it descended from a distinct H1 lineage that shared a common ancestor with the 1918 pandemic ~1907.”
“These results are consistent with an avian-to-human movement of H1 in the first decade of the 20th century,” the authors conclude. They say their theory fits with the age-related pattern of extensive morality in 20- to 40-year-olds, because people that age would have been primarily exposed to an H3N8 virus, which would afford little cross-immunity.
Previously, predictions that the 1918 strain was of avian origin and did not reassort with a human strain have predominated.