Update: influenza activity--United States, April-August 2009.

The first 2009 pandemic influenza A (H1N1) virus infections were identified in the United States in April 2009. By August, the cumulative number of infections in the United States was estimated to be at least 1 million. This report provides an overview of influenza activity during April-August 2009 and recommendations for the upcoming 2009-10 influenza season. Pandemic H1N1 influenza activity peaked in the United States during May and June and declined during July and early August. However, levels of influenza activity remained above normal for summer months, and focal outbreaks were reported throughout the summer. During the last 2 weeks of August, pandemic H1N1 influenza activity increased in certain areas of the United States. Clinicians and public health officials should be aware that these recent increases might signal an early start to the 2009-10 influenza season, with pandemic H1N1 influenza viruses predominating at least initially.