Context Respiratory viral infections are responsible for a large number of hospitalizations
in the United States each year.
Objective To estimate annual influenza-associated hospitalizations in the United
States by hospital discharge category, discharge type, and age group.
Design, Setting, and Participants National Hospital Discharge Survey (NHDS) data and World Health Organization
Collaborating Laboratories influenza surveillance data were used to estimate
annual average numbers of hospitalizations associated with the circulation
of influenza viruses from the 1979-1980 through the 2000-2001 seasons in the
United States using age-specific Poisson regression models.
Main Outcome Measures We estimated influenza-associated hospitalizations for primary and any
listed pneumonia and influenza and respiratory and circulatory hospitalizations.
Results Annual averages of 94 735 (range, 18 908-193 561) primary
and 133 900 (range, 30 757-271 529) any listed pneumonia and
influenza hospitalizations were associated with influenza virus infections.
Annual averages of 226 054 (range, 54 523-430 960) primary
and 294 128 (range, 86 494-544 909) any listed respiratory
and circulatory hospitalizations were associated with influenza virus infections.
Persons 85 years or older had the highest rates of influenza-associated primary
respiratory and circulatory hospitalizations (1194.9 per 100 000 persons).
Children younger than 5 years (107.9 primary respiratory and circulatory hospitalizations
per 100 000 persons) had rates similar to persons aged 50 through 64
years. Estimated rates of influenza-associated hospitalizations were highest
during seasons in which A(H3N2) viruses predominated, followed by B and A(H1N1)
seasons. After adjusting for the length of each influenza season, influenza-associated
primary pneumonia and influenza hospitalizations increased over time among
the elderly. There were no significant increases in influenza-associated primary
respiratory and circulatory hospitalizations after adjusting for the length
of the influenza season.
Conclusions Significant numbers of influenza-associated hospitalizations in the
United States occur among the elderly, and the numbers of these hospitalizations
have increased substantially over the last 2 decades due in part to the aging
of the population. Children younger than 5 years had rates of influenza-associated
hospitalizations similar to those among individuals aged 50 through 64 years.
These findings highlight the need for improved influenza prevention efforts
for both young and older US residents.