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

In collaboration with the World Health Organization (WHO) and its collaborating laboratories, National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories, state and local health departments, and a network of sentinel physicians, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in circulating strains of influenza viruses. This report summarizes influenza activity in the United States during September 30-November 24, 2001, when the viruses isolated most frequently were influenza A (H3N2). These viruses were well matched antigenically by the 2001-02 influenza A (H3N2) vaccine. Vaccine supplies are plentiful and influenza vaccine should continue to be offered during December and later.