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. 2021 Jan 15;11:631736. doi: 10.3389/fmicb.2020.631736

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Copyright © 2021 Piret and Boivin.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Timeline of influenza pandemics caused by the 1918 H1N1 virus and its descendants produced by reassortment of circulating strains with avian influenza viruses (AIV) and swine H1N1 viruses. The reassortment of genes is shown in parenthesis. The re-emergence of H1N1 virus in 1977 is also shown as it co-circulated with the H3N2 virus before being replaced by the H1N1pdm09. HA, hemagglutinin; NA, neuraminidase; NP, nucleoprotein; M, matrix proteins, PB1 polymerase; PB2 polymerase; PA polymerase; NS, non-structural proteins.