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Seasonal Influenza-Associated Hospitalizations in the United States
How many people in the United States are hospitalized with influenza in a typical year?
A study conducted by CDC and published in the Journal of American Medical Association (JAMA) in September 2004 provided information on the number of people in the United States that are hospitalized from influenza-related complications each year. The study was based on records from 1979 to 2001 from about 500 hospitals across the United States. The study concluded that, on average, more than 200,000 people in the United States are hospitalized each year for respiratory and heart conditions illnesses associated with influenza virus infections.
Is the number of people in the United States being hospitalized for flu complications each year increasing?
The results of a study conducted by CDC and published in the Journal of American Medical Association (JAMA) in September 2004 indicated an overall increasing trend in the number of flu-related hospitalizations in the United States each year. The study looked at hospital records from 1979 to 2001. In 1979, there were 120,929 flu-related hospitalizations. The number was lower in some years after that, but there was an overall upward trend. During the 1990s, the average number of people hospitalized was over 200,000 but individual seasons ranged from a low of 157,911 in 1990-91 to a high of 430,960 in 1997-98.
What was the previous estimate of the number of people in the United States hospitalized as a result of influenza?
In a paper published in 2000, CDC estimated that an average of 114,000 people were hospitalized as a result of influenza-associated infections each year.
What accounts for the increase in the number of people in the United States hospitalized for influenza each year?
The new estimates about the number of influenza-associated hospitalizations in the United States each year are higher for several reasons.
- The range of illnesses analyzed in the new study is broader than in the previous study. The new study includes respiratory and heart diseases associated with influenza infections. The earlier CDC study published in 2000 only analyzed pneumonia and influenza hospitalizations. When analyses were restricted only to pneumonia and influenza hospitalizations, however, there was still an increase in hospitalizations.
- Influenza A (H3N2) viruses predominated in several recent influenza seasons, and these viruses generally have been associated with higher numbers of serious illnesses than influenza A (H1N1) or influenza B viruses. The higher numbers of people hospitalized during H3N2 influenza seasons may have increased the average.
- The U.S. population is growing older and therefore, more vulnerable to developing severe complications from influenza.
- During the 1990s influenza viruses have either circulated or been detected for longer periods of time.
What else did the new study on influenza-associated hospitalizations find?
- The study found that while the number of influenza-associated hospitalizations is increasing, the rates of influenza associated hospitalizations were stable after accounting for the length of the influenza season.
- The study concluded that the rates of influenza-associated hospitalization during an influenza season are highest when influenza A (H3N2) viruses predominate compared to years when influenza A (H1N1) or influenza B viruses predominated.
- The study found that people 85 years and older have the highest rates of influenza-associated hospitalization.
- The study found that children younger than 5 years old had rates of hospitalization similar to people 50 to 64 years of age.
These findings support previously published data.
What is the main lesson learned from this study?
Influenza is associated with large numbers of hospitalizations. The results of this study demonstrate the substantial health impact of influenza and underscore the need to ensure vaccination of people at increased risk of serious influenza complications, especially people 65 years and older.
Can you provide more information about the study on influenza-associated hospitalizations?
The study is entitled “Influenza-Associated Hospitalizations in the United States 1979 Through the 2000-2001 Respiratory Seasons.” It appeared in the Journal of American Medical Association’s September 14, 2004 issue (volume 292, no. 11). The study was co-authored by 7 CDC staff members.
- Page last reviewed September 18, 2006
- Page last modified September 22, 2004
- Content Source: Coordinating Center for Infectious Diseases (CCID)
- National Center for Immunization and Respiratory Diseases (NCIRD)
