A Review of Adult Mortality Due to 2009 Pandemic (H1N1) Influenza A in California

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.

The characteristics, clinical manifestations and outcomes of pandemic influenza A (H1N1) 2009 in the elderly

“…The variations between these results can be justified by differences in the clinical severity of disease according to geographical region, climate and host adaptation 6 . The average time from symptom onset to death in our study was 13.9 days, which was similar to the time period of 14 days observed in California 17 . The rate of oseltamivir treatment was 88.6% in our study and was similar to the treatment rate of 80% observed in another study 17 .…”

“…The average time from symptom onset to death in our study was 13.9 days, which was similar to the time period of 14 days observed in California 17 . The rate of oseltamivir treatment was 88.6% in our study and was similar to the treatment rate of 80% observed in another study 17 . Early treatment within 2 days after the onset of symptoms was significantly associated with a lower risk of emergency admissions and death in hospitalized patients who contracted the 2009 Pandemic Influenza A (H1N1) 20,21 .…”

“…The elderly in this study, divided by age, showed mortality rates lower than those found in other studies conducted in California, which showed rates of 37.8%, 42.3% and 36% for ages 60-69, 70-79 and over 80 years of age, respectively 17 . Nevertheless, other studies have reported mortality rates of 2.2% 18 and 1.7% 19 .…”

“…Our results suggest benefits that elderly persons may receive from vaccination. However, despite the reduction in hospitalization and mortality rates observed among vaccinated patients, the overall effectiveness of vaccination in the elderly is uncertain 17 . A large, randomized, controlled trial using vaccinated subjects slightly older and with higher prevalence rates of all the baseline medical conditions except dementia or stroke, found that vaccination for seasonal flu was associated with a 27% reduction in the risk of hospitalization for pneumonia or influenza (adjusted odds ratio, 0.73; 95% confidence interval [CI], 0.68 to 0.77) and a 48% reduction in the risk of death (adjusted odds ratio, 0.52; 95% CI, 0.50 to 0.55) 23 .…”

Infection with Seasonal Influenza Virus Elicits CD4 T Cells Specific for Genetically Conserved Epitopes That Can Be Rapidly Mobilized for Protective Immunity to Pandemic H1N1 Influenza Virus

“…For example, clinical and epidemiological studies of the pandemic H1N1 virus infections worldwide suggested that rates of infection with the pandemic H1N1 2009 influenza virus differed significantly in different age groups, with children and young adults disproportionately susceptible to infection (4,24). Depending on the study and region analyzed, individuals under the age of 25 years represented 45% to 60% of infected subjects, though the pathogenic effects of H1N1 virus infection were most pronounced in individuals more than 60 years old (4,36). These findings, as well as recent immunological studies from our laboratory and other laboratories (11,17,20,22,25,33,39,48,51,52,55,61,62), suggest that previous encounters with vaccines or viruses provide immunological advantages and immunological memory in the population despite the "serological distance" between the hemagglutinin (HA) and neuraminidase (NA) proteins of seasonal and pandemic strains.…”