Complications among adults hospitalized with influenza: a comparison of seasonal influenza and the 2009 H1N1 pandemic.
BACKGROUND
Persons with influenza can develop complications that result in hospitalization and death. These are most commonly respiratory related, but cardiovascular or neurologic complications or exacerbations of underlying chronic medical conditions may also occur. Patterns of complications observed during pandemics may differ from typical influenza seasons, and characterizing variations in influenza-related complications can provide a better understanding of the impact of pandemics and guide appropriate clinical management and planning for the future.


METHODS
Using a population-based surveillance system, we compared clinical complications using International Classification of Diseases, Ninth Revision (ICD-9) discharge diagnosis codes in adults hospitalized with seasonal influenza (n = 5270) or 2009 pandemic influenza A(H1N1) (H1N1pdm09; n = 4962).


RESULTS
Adults hospitalized with H1N1pdm09 were younger (median age, 47 years) than those with seasonal influenza (median age, 68 years; P < .01), and differed in the frequency of certain underlying medical conditions. Whereas there was similar risk for many influenza-associated complications, after controlling for age and type of underlying medical condition, adults hospitalized with H1N1pdm09 were more likely to have lower respiratory tract complications, shock/sepsis, and organ failure than those with seasonal influenza. They were also more likely to be admitted to the intensive care unit, require mechanical ventilation, or die. Young adults, in particular, had 2-4 times the risk of severe outcomes from H1N1pdm09 than persons of the same ages with seasonal influenza.


CONCLUSIONS
Although H1N1pdm09 was thought of as a relatively mild pandemic, these data highlight the impact of the 2009 pandemic on the risk of severe influenza, especially among younger adults, and the impact this virus may continue to have.
Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia.
Carrie
C
Reed
Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia.
Sandra S
SS
Chaves
Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia.
Alejandro
A
Perez
Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia.
Tiffany
T
D'Mello
California Emerging Infections Program (EIP), Oakland.
Pamala
P
Daily Kirley
Colorado Department of Public Health and Environment, Denver.
Deborah
D
Aragon
Connecticut EIP, Yale School of Public Health, New Haven.
James I
JI
Meek
Emory University School of Medicine and Atlanta Veterans Affairs Medical Center, Atlanta, Georgia.
Monica M
MM
Farley
Maryland EIP, Maryland Department of Health and Mental Hygiene, Baltimore.
Patricia
P
Ryan
Minnesota EIP, Minnesota Department of Health, St Paul.
Ruth
R
Lynfield
Minnesota EIP, Minnesota Department of Health, St Paul.
Craig A
CA
Morin
New Mexico EIP, New Mexico Department of Health, Santa Fe.
Emily B
EB
Hancock
Department of Medicine, University of Rochester School of Medicine and Dentistry.
Nancy M
NM
Bennett
New York State Department of Health, Albany.
Shelley M
SM
Zansky
Oregon Public Health Division, Portland.
Ann
A
Thomas
Tennesee EIP, Tennessee Department of Health, Nashville.
Mary Louise
ML
Lindegren
Tennesee EIP, Tennessee Department of Health, Nashville.
William
W
Schaffner
Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia.
Lyn
L
Finelli
D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006760:Hospitalization; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification; D007251:Influenza, Human / Q000150:complications* / Q000453:epidemiology / Q000473:pathology / Q000821:virology*; D008297:Male; D008875:Middle Aged
hospitalization; pandemic influenza; pneumonia; seasonal influenza