Respiratory syncytial virus activity - United States, July 2008-December 2009.

Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis and pneumonia in children aged <1 year worldwide. Each year in the United States, an estimated 75,000-125,000 infants are hospitalized with RSV. Among adults aged >65 years, an estimated 177,000 hospitalizations and 14,000 deaths a year have been attributed to RSV infections. In temperate climates, the RSV season generally begins during the fall and continues through the winter and spring, but the exact timing of RSV circulation varies by location and year. In the United States, data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) are used to monitor the seasonal occurrence of RSV. During the 2008-09 season, onset occurred from mid-October to late December in the 10 U.S. Department of Health and Human Services (HHS) regions, excluding Florida, which had onset in mid-July. Season offset in all regions occurred from mid-February to mid-April. Florida is reported separately because it has an earlier season onset and longer duration than the rest of the country. During the current 2009-10 season, onset occurred in all 10 HHS regions by February 20, 2010. These patterns are similar to previous years and confirm differences in RSV seasonal characteristics across regions. Knowledge of RSV seasonality can be used by clinicians and public health officials to determine when to consider RSV as a cause of acute respiratory illnesses and when to provide RSV immunoprophylaxis to children at high risk for serious disease.