Update: influenza activity--United States and worldwide, 2005-06 season, and composition of the 2006-07 influenza vaccine.

During the 2005-06 influenza season, influenza A (H1N1), A (H3N2), and B viruses cocirculated worldwide. In the United States, influenza A (H3N2) viruses predominated overall, but influenza B viruses were isolated more frequently than influenza A viruses late in the season. Influenza activity in the United States peaked in early March, and the number of pneumonia and influenza deaths did not exceed the epidemic threshold. Worldwide, influenza B viruses were the most commonly reported influenza type in Europe; influenza A (H1N1) and influenza B viruses predominated in Asia. Through June 13, 2006, outbreaks of influenza A (H5N1) viruses (avian influenza) among migratory birds and poultry flocks were associated with severe human illness or death in 10 countries (Azerbaijan, Cambodia, China, Djibouti, Egypt, Indonesia, Iraq, Thailand, Turkey, and Vietnam). This report summarizes influenza activity in the United States and worldwide during the 2005-06 influenza season and describes composition of the 2006-07 influenza vaccine.