Adult hospitalizations for laboratory-positive influenza during the 2005-2006 through 2007-2008 seasons in the United States.

Background: Rates of influenza-associated hospitalizations in the United States have been estimated using modeling techniques with data from pneumonia and influenza hospitalization discharge diagnoses, but they have not been directly estimated from laboratory-positive cases.

Methods: We calculated overall, age-specific, and site-specific rates of laboratory-positive, influenza-associated hospitalization among adults and compared demographic and clinical characteristics and outcomes of hospitalized cases by season with use of data collected by the Emerging Infections Program Network during the 2005-2006 through 2007-2008 influenza seasons.

Results: Overall rates of adult influenza-associated hospitalization per 100,000 persons were 9.9 during the 2005-2006 season, 4.8 during the 2006-2007 season, and 18.7 during the 2007-2008 season. Rates of hospitalization varied by Emerging Infections Program site and increased with increasing age. Higher overall and age-specific rates of hospitalization were observed during influenza A (H3) predominant seasons and during periods of increased circulation of influenza B. More than 80% of hospitalized persons each season had > or =1 underlying medical condition, including chronic cardiovascular and metabolic diseases.

Conclusions: Rates varied by season, age, geographic location, and type/subtype of circulating influenza viruses. Influenza-associated hospitalization surveillance is essential for assessing the relative severity of influenza seasons over time and the burden of influenza-associated complications.