Update: influenza activity--worldwide, April-September 1998.

In collaboration with the World Health Organization (WHO), the WHO international network of approximately 110 collaborating laboratories in 83 countries, and U.S. state and local health departments, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses. During October 1997-April 1998. influenza activity was moderate to severe in the Northern Hemisphere. Influenza A(H3N2) viruses were predominant, but influenza A(H1N1) viruses were associated with outbreaks, and influenza B viruses were identified sporadically in all regions. Since April 1998, increased influenza outbreak and epidemic level activity, primarily associated with influenza A(H3N2), has been reported in the Southern Hemisphere. This report summarizes worldwide influenza activity during April-September 1998 and the antigenic characteristics of influenza isolates collected during April-September.