Figure 1. RNA Divergence of Influenza H1N1 Pandemic Strain, 2009–2013.

Since April 2009, the coding region of the hemagglutinin segment of the influenza H1N1 pandemic strain has been accumulating nucleotide mutations at a rate of approximately 5.68 mutations per annum (A). This contrasts with the coding regions for the neuraminidase and nucleoprotein segments which have been accumulating mutations at a rate of only 3.56 and 3.81 mutations per annum, respectively (B).