Update: influenza activity - United States, 2011-12 season and composition of the 2012-13 influenza vaccine.

During the 2011-12 influenza season in the United States, influenza activity* occurred at low levels during October through December and increased in January and February before peaking in mid-March. Influenza A (H3N2) viruses predominated overall, but influenza A (H1N1)pdm09 (pH1N1) and influenza B viruses also circulated widely. This influenza season was mild compared with recent years, with a lower percentage of outpatient visits for influenza-like illness (ILI),† lower rates of hospitalizations, and fewer deaths attributed to pneumonia and influenza. This report summarizes influenza activity in the United States during the 2011-12 influenza season (October 2, 2011-May 19, 2012) and reports the recommendations for the components of the 2012-13 Northern Hemisphere influenza vaccine.