Update: influenza activity--United States, 1997-98 season.

In collaboration with the World Health Organization (WHO), its collaborating laboratories, and state and local health departments, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses. This report summarizes influenza surveillance in the United States from September 28, 1997, to January 10, 1998, which indicates that influenza activity, predominantly attributable to influenza A(H3N2) viruses, increased from mid-December through early January, and two antigenically related but distinguishable strains of influenza A(H3N2) viruses, A/Nanchang/933/95-like and A/Sydney/05/97-like, have been identified. Few influenza B isolates have been identified.