Update: Influenza activity--United States and worldwide, 2006-07 season, and composition of the 2007-08 influenza vaccine.

During the 2006-07 season, influenza activity peaked in mid-February in the United States and was associated with less mortality and lower rates of pediatric hospitalizations than during the previous three seasons. In the United States, influenza A (H1) viruses predominated overall, but influenza A (H3) viruses were isolated more frequently than influenza A (H1) viruses late in the season. Although influenza A (H1), A (H3), and B viruses cocirculated worldwide, influenza A (H3) viruses were the most commonly reported type in Europe and Asia. Sporadic cases of avian influenza A (H5N1) virus infections associated with severe illness or death were reported among humans in Cambodia, China, Egypt, Indonesia, Laos, Nigeria, and Viet Nam. This report summarizes influenza activity in the United States and worldwide during the 2006-07 influenza season (October 1, 2006-May 19, 2007) and describes the composition of the 2007-08 influenza vaccine.