Update: influenza activity--United States, September 28, 2008-April 4, 2009, and composition of the 2009-10 influenza vaccine.

This report summarizes U.S. influenza activity from September 28, 2008, the start of the 2008-09 influenza season, through April 4, 2009, and reports on the 2009-10 influenza vaccine strain selection. Low levels of influenza activity were reported from October through early January. Activity increased from mid-January and peaked in mid-February. Influenza A (H1N1) viruses have predominated overall this season, but influenza B viruses have been isolated more frequently than influenza A viruses since mid-March. Widespread oseltamivir resistance was detected among circulating influenza A (H1N1) viruses and a high level of adamantane resistance was identified among influenza A (H3N2) viruses.