The 2009 H1N1 influenza outbreak in its historical context

Phylogenetic tree of H1 haemagglutinin sequences in influenza A H1N1 strains infecting humans from 1918 to the present day. 3 clades can be seen: (1) the 1918 pandemic strain which is an outlier; (2) strains circulating seasonally since at least the 1930s (lower of 2 major clades); (3) zoonotic strains (upper clade). The positions of the current outbreak and the most recent vaccine strains are indicated. Tree drawn using the NCBI Influenza Virus Resource tools: (http://www.ncbi.nlm.nih.gov/genomes/FLU/Database).

Phylogenetic tree of H1 haemagglutinin sequences in influenza A H1N1 strains infecting pigs from 1930 to the present day. 3 clades can be seen: (1) some early swine flu strains from 1930 to 1945, which are outliers; (2) “Classic” North American swine flu (lower of 2 major clades); (3) atypical swine flu (upper clade, in yellow), including “Eurasian” swine flu and some further atypical strains, including one from 1930 and a group from 2004 to 2006. Tree drawn using the NCBI Influenza Virus Resource tools: (http://www.ncbi.nlm.nih.gov/genomes/FLU/Database).