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ML tree (unrooted) of 652 HA1 sequences of seasonal H1N1 virus sampled globally during the period from 2008 to 2009. The seven viruses sampled from New York State during the period 27 April to 26 May are shaded red, with each likely introduction event marked by a square. A possible transmission cluster is denoted with an asterisk; the bootstrap value for this node is also shown.
