Factors associated with death or hospitalization due to pandemic 2009 influenza A(H1N1) infection in California.
BACKGROUND
Pandemic influenza A(H1N1) emerged rapidly in California in April 2009. Preliminary comparisons with seasonal influenza suggest that pandemic 2009 influenza A(H1N1) disproportionately affects younger ages and causes generally mild disease.


OBJECTIVE
To describe the clinical and epidemiologic features of pandemic 2009 influenza A(H1N1) cases that led to hospitalization or death.


METHODS
Statewide enhanced public health surveillance of California residents who were hospitalized or died with laboratory evidence of pandemic 2009 influenza A(H1N1) infection reported to the California Department of Public Health between April 23 and August 11, 2009.


METHODS
Characteristics of hospitalized and fatal cases.


RESULTS
During the study period there were 1088 cases of hospitalization or death due to pandemic 2009 influenza A(H1N1) infection reported in California. The median age was 27 years (range, <1-92 years) and 68% (741/1088) had risk factors for seasonal influenza complications. Sixty-six percent (547/833) of those with chest radiographs performed had infiltrates and 31% (340/1088) required intensive care. Rapid antigen tests were falsely negative in 34% (208/618) of cases evaluated. Secondary bacterial infection was identified in 4% (46/1088). Twenty-one percent (183/884) received no antiviral treatment. Overall fatality was 11% (118/1088) and was highest (18%-20%) in persons aged 50 years or older. The most common causes of death were viral pneumonia and acute respiratory distress syndrome.


CONCLUSIONS
In the first 16 weeks of the current pandemic, the median age of hospitalized infected cases was younger than is common with seasonal influenza. Infants had the highest hospitalization rates and persons aged 50 years or older had the highest mortality rates once hospitalized. Most cases had established risk factors for complications of seasonal influenza.
Louie
Janice K
JK
California Department of Public Health, 850 Marina Bay Pkwy, Richmond, CA 94804, USA. Janice.Louie@cdph.ca.gov
Acosta
Meileen
M
Winter
Kathleen
K
Jean
Cynthia
C
Gavali
Shilpa
S
Schechter
Robert
R
Vugia
Duc
D
Harriman
Kathleen
K
Matyas
Bela
B
Glaser
Carol A
CA
Samuel
Michael C
MC
Rosenberg
Jon
J
Talarico
John
J
Hatch
Douglas
D
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002140:California / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D004196:Disease Outbreaks; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000150:complications* / Q000453:epidemiology / Q000401:mortality*; D008297:Male; D008875:Middle Aged; D011024:Pneumonia, Viral / Q000150:complications / Q000401:mortality; D011159:Population Surveillance; D012128:Respiratory Distress Syndrome / Q000150:complications / Q000401:mortality; D012307:Risk Factors; D015163:Superinfection / Q000150:complications / Q000401:mortality; D016019:Survival Analysis; D055815:Young Adult