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Seasonal Influenza-Associated Hospitalizations in the United States

Questions & Answers

How many people in the United States are hospitalized with seasonal influenza in a typical year?

An online report published on December 9, 2016 estimated that from 2010-2011 to 2015-2016, flu-related hospitalizations in the United States ranged from a low of 140,000 (during 2011-2012) to a high of 710,000 (during 2014-2015). During the 2015-2016 flu season, CDC estimated that 310,000 people were hospitalized for flu-related illness.

What were the previous estimates of the number of people in the United States hospitalized as a result of seasonal influenza?

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 were 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.

Prior to that, in a paper published in 2000(1), CDC estimated that an average of 114,000 people were hospitalized as a result of seasonal influenza-associated infections each year.

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Why are the current estimates of influenza-associated hospitalizations different from previous estimates?

The estimated number of Influenza hospitalizations has always varied from season to season. In the past, CDC had referred to an average number of hospitalizations that was estimated based on statistical models using hospital discharge records from 1979 to 2001 from selected hospitals across the United States.

The most recent estimates use surveillance data on patients hospitalized with laboratory-confirmed influenza in geographically distributed areas in the United States. This method has been applied to influenza seasons since 2010-2011 and a five-year range of influenza-associated hospitalizations has been estimated from the 2010-2011 through 2014-2015 influenza seasons.

Estimates of influenza-associated hospitalizations using surveillance data are straightforward, use directly observed hospitalization rates, and can be updated annually, whereas there are delays in the availability of large administrative databases that go into estimates based on statistical modeling.

Additionally, CDC believes that the estimate of a five-year range better represents the variability of influenza seasons than an average estimate.

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What is the main lesson learned from the updated estimates of influenza-associated hospitalization?

Seasonal influenza is associated with large numbers of hospitalizations. The results of this online report and previous studies 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.

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Can you provide more information about the 2000 and 2004 studies on seasonal influenza-associated hospitalizations?

The study published in 2000 is entitled "The Impact of Influenza Epidemics on Hospitalizations." It appeared in the Journal of Infectious Diseases' March, 2000 issue (volume 191, no. 3).

The study published in 2004 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).

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Can you provide more information about the 2016 online report on seasonal influenza-associated hospitalizations?

The range of estimated flu-related hospitalizations from 2010-2011 to 2014-2015 appeared online in the 2015-2016 Estimates Influenza Illnesses, Medical visits, Hospitalizations, and Deaths Averted by Vaccination in the United States.  At the end of the online report, there are references to other publications that provide even more information on this methodology.

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References

  1. Simonsen L, Fukuda K, Schonberger LB, Cox NJ. The impact of influenza epidemics on hospitalizations. J Infect Dis. 2000;181(3):831-7. Available from: https://www.ncbi.nlm.nih.gov/pubmed/10720501

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