Update: influenza activity--worldwide, May-September 1999.

In collaboration with the World Health Organization (WHO), the WHO international network of 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. From October 1998 through April 1999, influenza activity was moderate to severe in the Northern Hemisphere. Influenza A(H3N2) viruses predominated but influenza type B viruses were isolated more frequently than influenza A in some countries. Influenza A(H1N1) viruses were isolated from sporadic cases in Asia, Europe, and North America, and from outbreaks in South America. Since May 1999, influenza activity associated primarily with influenza A(H3N2) viruses has peaked and is declining in the Southern Hemisphere. This report summarizes worldwide influenza activity during May-September 1999 and the antigenic characteristics of influenza isolates collected during May-August 1999.