Disease Burden of Influenza
The severity of influenza disease in the United States can vary widely and is determined by a number of things including the characteristics of circulating viruses, the timing of the season, how well the vaccine is working to protect against illness, and how many people got vaccinated. CDC tracks severity principally through its national Influenza Surveillance System that monitors key indicators like the percentage of deaths resulting from pneumonia or influenza, rates of influenza-associated hospitalizations, pediatric deaths and the percentage of visits to outpatient clinics for influenza-like illness. In addition to using surveillance data, CDC uses mathematical models to fill in the picture of the disease burden and the impact of influenza immunization programs. Models are used to augment surveillance because most of the surveillance systems only look at portions of the U.S. population and in some cases there can be significant under-reporting of influenza deaths and hospitalizations. This page includes links to key resources on the burden of influenza.