Update: influenza activity--United States and worldwide, May-September 2001

During October 2000-May 2001, influenza A (H1N1), A (H3N2), and B viruses were identified in the Northern Hemisphere. Influenza A (H1N1) and B viruses circulated widely; influenza A (H3N2) viruses were reported infrequently and were not associated with widespread outbreaks in any country during October 2000-May 2001. Since May 2001, influenza A (H1N1) and B viruses have predominated in Asia and Oceania; influenza A (H3N2) and B viruses have predominated in Africa and South America. This report summarizes influenza activity in the United States* (1) and worldwide during May-September 25; influenza A (H1N1), A (H3N2), and B viruses continued to circulate worldwide and were associated with mild to moderate levels of activity. This activity underscores the need to follow the recommendations of the Advisory Committee on Immunization Practices (ACIP) (2,3) for the timely vaccination of persons at high risk for influenza-related complications.