Update: influenza activity--worldwide, March-August 1997.

During the October 1996-March 1997 influenza epidemic season, influenza activity was moderate to severe in the Northern Hemisphere. In Europe, Japan, and North America, influenza A(H3N2) viruses predominated, but influenza B viruses were more commonly isolated by the end of the season. In contrast, in some countries in the World Health Organization (WHO) region of Asia (e.g., China, Iran, and Israel), influenza B viruses were isolated more frequently than influenza A(H3N2) viruses. Influenza A(H1N1) viruses were isolated infrequently worldwide, except in Europe, where 14 countries reported sporadic isolations, and a late-season outbreak in April affected children in the Czech Republic. Since March 1997, influenza activity has increased in the Southern Hemisphere, and outbreaks and epidemic level activity have been associated with both influenza A(H3N2) and influenza B viruses. This report summarizes worldwide influenza activity during March-August 1997, as reported through WHO, the WHO international network of collaborating laboratories, and U.S. state and local health departments, and characterizes influenza isolates collected during March-July.