Influenza--United States, 1989-90 and 1990-91 seasons.

During the 1989-90 influenza season, 98% of all influenza viruses isolated in the United States and reported to CDC were influenza A. Almost all those that were antigenically characterized were similar to influenza A/Shanghai/11/87(H3N2), a component of the 1989-90 influenza vaccine. Regional and widespread influenza activity began to be reported in late December 1989, peaked in mid-January 1990, and declined rapidly through early April 1990. Most of the outbreaks reported to CDC were among nursing-home residents. Considerable influenza-associated mortality was reflected in the percentage of deaths due to pneumonia and influenza (P&I) reported through the CDC 121 Cities Surveillance System from early January through early April. More than 80% of all reported P&I deaths were among persons greater than or equal to 65 years. In contrast to the predominance of influenza A during 1989-90, during the 1990-91 influenza season 86% of all influenza virus isolations reported were influenza B. Widespread influenza activity was reported from mid-January through April 1991, with regional activity extending into May. Outbreaks were reported primarily among schoolchildren, and no evidence of excess influenza-associated mortality was found. Almost all the influenza B isolates tested were related to influenza B/Yamagata/16/88, a component of the 1990-91 influenza vaccine, but were antigenically closer to B/Panama/45/90, a minor variant.