Questions & Answers

Seasonal Influenza-Associated Hospitalizations in the United States

How many people in the United States are hospitalized with seasonal 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 seasonal 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 seasonal influenza virus infections.

Is the number of people in the United States being hospitalized for seasonal 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 seasonal 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 seasonal influenza?

In a paper published in 2000, CDC estimated that an average of 114,000 people were hospitalized as a result of seasonal influenza-associated infections each year.

Top

What accounts for the increase in the number of people in the United States hospitalized for seasonal influenza each year?

The new estimates about the number of seasonal influenza-associated hospitalizations in the United States each year are higher for several reasons.

1. The range of illnesses analyzed in the newer study is broader than in the previous study. The more recent 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.
2. Influenza A (H3N2) viruses predominated in several recent influenza seasons, and these viruses generally have been associated with higher numbers of serious illnesses than seasonal influenza A (H1N1) or influenza B viruses. The higher numbers of people hospitalized during H3N2 influenza seasons may have increased the average.
3. The U.S. population is growing older and therefore, more vulnerable to developing severe complications from seasonal influenza.
4. During the 1990s, seasonal influenza viruses either circulated or were detected for longer periods of time; a longer season would result in greater numbers of people being hospitalized.

What else did the new study on seasonal influenza-associated hospitalizations find?

• The study found that while the number of seasonal influenza-associated hospitalizations is increasing, the rates of seasonal influenza associated hospitalizations were stable after accounting for the length of the influenza season.
• The study concluded that the rates of seasonal influenza-associated hospitalization during an influenza season are highest when seasonal influenza A (H3N2) viruses predominate compared to years when seasonal influenza A (H1N1) or influenza B viruses predominated.
• The study found that people 85 years and older have the highest rates of seasonal 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?

Seasonal influenza is associated with large numbers of hospitalizations. The results of this study demonstrate the substantial health impact of seasonal 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 seasonal 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.

Top