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Genetic relationships of human, swine, and avian influenza viruses for H1.
Names represent viruses all containing an H1 and different neuraminidase types (as indicated). Year of isolation is indicated. The 2007 WHO recommended H1N1 vaccine strain: A/Brisbane/59/2007 (H1N1) is underlined. The 2009 pandemic H1N1 (pH1N1) can be found at the top branch of the tree. Scale bar indicates amino acid substitutions per site.
