Update: influenza activity - United States, August 30, 2009-March 27, 2010, and composition of the 2010-11 influenza vaccine.

The emergence and spread of 2009 pandemic influenza A (H1N1) virus resulted in substantial influenza activity in the United States throughout the summer and fall months of 2009, with activity peaking in late October. Activity declined beginning in November 2009 but continued at lower levels through March 2010. The 2009 H1N1 virus remained the dominant circulating influenza virus throughout the season; <1% of characterized viruses were seasonal influenza A (H1), A (H3), and influenza B viruses. This report summarizes U.S. influenza activity from August 30, 2009, the start of the 2009-10 influenza season, through March 27, 2010, and also reports on the 2010-11 Northern Hemisphere influenza vaccine strain selection.