Update: Influenza activity--United States, 2001-02 season.

Although data collected from the four components of the CDC influenza surveillance system are preliminary, national influenza activity appears to have peaked during the week ending February 23, 2002 (week 8). During the 2000-01 and 1999-2000 influenza seasons, peak activity occurred during week 4 and week 51, respectively. The viruses most commonly isolated during the 2001-02 season have been influenza A (H3N2). These viruses were well-matched antigenically by the 2001-02 influenza A (H3N2) strain in the vaccine. This report summarizes influenza activity in the United Statest during September 30, 2001-March 23, 2002, and updates previous summaries from this season.