Update: influenza activity - United States, September 28, 2008--January 31, 2009.

From September 28, 2008, to January 31, 2009, influenza activity remained low in the United States but began to increase at the end of January. Thus far during the 2008-09 influenza season, influenza A viruses have predominated and are antigenically related to the 2008-09 influenza vaccine strains. Oseltamivir resistance has been detected in nearly all of the influenza A (H1N1) viruses tested so far during the 2008-09 season, with high levels of adamantane resistance among influenza A (H3N2) viruses. This report summarizes U.S. influenza activity since the last update and reviews interim recommendations for the use of influenza antiviral medications.