Update: influenza activity--United States and worldwide, 1999-2000 season, and composition of the 2000-01 influenza vaccine.

Influenza A (H3N2) viruses were the predominant viruses isolated in the United States and worldwide during 1999-2000. This was the third consecutive year that influenza A/Sydney/05/97-like (H3N2) viruses were the most prevalent viruses isolated in the United States. Influenza activity in the United States was similar to the previous two seasons, although mortality measurements attributed to pneumonia and influenza (P&I) were unusually high. Overall, the 1999-2000 influenza vaccine was well matched to circulating influenza viruses. The 2000-01 influenza season will be the first for which influenza vaccination is recommended for all persons aged > or =50 years. This report summarizes surveillance for influenza in the United States and worldwide during the 1999-2000 influenza season, describes the composition of the 2000-01 influenza vaccine, and highlights changes in the recommendations for prevention and control of influenza.