Abstract
BackgroundWhile children and young adults had the highest attack rates due to 2009 pandemic (H1N1) influenza A (2009 H1N1), studies of hospitalized cases noted high fatality in older adults. We analyzed California public health surveillance data to better characterize the populations at risk for dying due to 2009 H1N1.Methods and FindingsA case was an adult ≥20 years who died with influenza-like symptoms and laboratory results indicative of 2009 H1N1. Demographic and clinical data were abstracted from medical records using a standardized case report form. From April 3, 2009 – August 10, 2010, 541 fatal cases ≥20 years with 2009 H1N1 were reported. Influenza fatality rates per 100,000 population were highest in persons 50–59 years (3.5; annualized rate = 2.6) and 60–69 years (2.3; annualized rate = 1.7) compared to younger and older age groups (0.4–1.9; annualized rates = 0.3–1.4). Of 486 cases hospitalized prior to death, 441 (91%) required intensive care unit (ICU) admission. ICU admission rates per 100,000 population were highest in adults 50–59 years (8.6). ICU case-fatality ratios among adults ranged from 24–42%, with the highest ratios in persons 70–79 years. A total of 425 (80%) cases had co-morbid conditions associated with severe seasonal influenza. The prevalence of most co-morbid conditions increased with increasing age, but obesity, pregnancy and obstructive sleep apnea decreased with age. Rapid testing was positive in 97 (35%) of 276 tested. Of 482 cases with available data, 384 (80%) received antiviral treatment, including 49 (15%) of 328 within 48 hours of symptom onset.ConclusionsAdults aged 50–59 years had the highest fatality due to 2009 H1N1; older adults may have been spared due to pre-existing immunity. However, once infected and hospitalized in intensive care, case-fatality ratios were high for all adults, especially in those over 60 years. Vaccination of adults older than 50 years should be encouraged.
Highlights
The 2009 pandemic (H1N1) influenza A virus (2009 H1N1) emerged in April 2009 to cause a global pandemic that lasted 16 months [1]
Influenza fatality rates were highest in persons 50–59 years (3.5; annualized rate 2.6) and 60–69 years (2.3; annualized rate 1.7) and lower (0.4– 1.9; annualized rates 0.3–1.4) in younger and older age groups
intensive care unit (ICU) admission rates per 100,000 population were highest in adults 50–59 years of age (8.6)
Summary
The 2009 pandemic (H1N1) influenza A virus (2009 H1N1) emerged in April 2009 to cause a global pandemic that lasted 16 months [1]. The highest attack rates were reported among children and young adults; 60% of laboratory-confirmed infections and 32–45% of hospitalized cases in the United States occurred in persons under 18 years of age, and cases younger than 65 years accounted for approximately 90% of deaths [1]. These preliminary data led to initial ACIP recommendations in July 2009 to target limited initial supplies of H1N1 vaccine to persons younger than age 65 years [2]; these recommendations have since been updated [3]. We analyzed California public health surveillance data to better characterize the populations at risk for dying due to 2009 H1N1
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