Influenza activity--United States, 2012-13 season and composition of the 2013-14 influenza vaccine.

During the 2012-13 influenza season in the United States, influenza activity* increased through November and December before peaking in late December. Influenza A (H3N2) viruses predominated overall, but influenza B viruses and, to a lesser extent, influenza A (H1N1)pdm09 (pH1N1) viruses also were reported in the United States. This influenza season was moderately severe, with a higher percentage of outpatient visits for influenza-like illness (ILI), higher rates of hospitalization, and more reported deaths attributed to pneumonia and influenza compared with recent years. This report summarizes influenza activity in the United States during the 2012-13 influenza season (September 30, 2012-May 18, 2013) as of June 7, 2013, and reports the recommendations for the components of the 2013-14 Northern Hemisphere influenza vaccine.