Update: influenza activity--United States, 2010-11 season, and composition of the 2011-12 influenza vaccine.

During the 2010-11 influenza season, influenza activity* first began to increase in the southeastern United States, and peaked nationally in early February. Compared with the previous pandemic year (2009-10), higher rates of hospitalization were observed for persons aged≥65 years during the 2010-11 season, whereas lower hospitalization rates were observed in younger populations than during the pandemic year. Overall, the percentages of outpatient visits for influenza-like illness (ILI) were lower during the 2010-11 season than the 2009-10 pandemic influenza season. In the United States, influenza A (H3N2) remained the predominant virus throughout the season; however, 2009 influenza A (H1N1) and influenza B viruses also circulated, and the predominant virus varied by U.S. Department of Health and Human Service (HHS) region and week. This report summarizes influenza activity in the United States during the 2010-11 influenza season (October 3, 2010-May 21, 2011) and describes the components of the 2011-12 Northern Hemisphere influenza vaccine.