Update: Influenza activity - United States, October 2, 2011-February 11, 2012.

This report summarizes U.S. influenza activity* since the beginning of the 2011-12 influenza season (October 2, 2011) and updates the previous report. From October through early January, influenza activity remained low throughout the United States. Activity increased slightly in early February 2012, but remains low. Influenza A (H1N1)pdm09 (pH1N1), influenza A (H3N2), and influenza B viruses all have been identified thus far this influenza season, and the majority of viruses in circulation are antigenically similar to strains included in the 2011-12 vaccine.