The influenza A (H1N1)pdm09 virus (H1N1pdm09) began to circulate in the United States in April 2009,1 and in June 2009 the World Health Organization declared a global influenza pandemic. In the United States, a small focal pandemic wave in the spring was followed by a very large wave of widespread illness in all parts of the country in the fall.2 Although for most persons infected, influenza is a mild, self-limited illness, influenza epidemics and pandemics are associated with substantial morbidity and mortality in a population. This is particularly true in vulnerable populations, such as pregnant women, young children, the elderly, and those with certain underlying medical conditions.3 Identifying persons at greatest risk for severe outcomes from influenza helps to guide prevention and control efforts.4
The Centers for Disease Control and Prevention (CDC) monitored severe outcomes from H1N1pdm09 infection in the United States throughout the pandemic through individual reports and aggregate reporting by jurisdictions.5–7 A study of the characteristics of H1N1pdm09 influenza-associated deaths that occurred during April–July 2009 in the United States reported that 76% of deaths occurred in persons aged 18–64 years and 78% had underlying medical conditions.5 Chronic lung disease, including asthma, was the most common condition reported among cases aged ≥18 years (39%), while neurologic diseases were the most common conditions reported among cases aged <18 years (54%). This and other reports suggested that the age groups most at risk of severe H1N1pdm09 disease differed from those for seasonal influenza where the highest number of deaths is seen in persons aged ≥65 years.3,6–8 Based on experience from past pandemics, there was concern that in subsequent waves the H1N1pdm09 virus would become more virulent and cause more severe disease.9 Thus, surveillance for severe and fatal H1N1pdm09 cases was necessary at the start of the fall wave to detect potential changes in the epidemiologic patterns observed during the early months of the pandemic and ensure that vaccination and treatment recommendations remained targeted to those at greatest risk.5 To monitor characteristics associated with death from H1N1pdm09, a nationally representative study of deaths that occurred during September–October 2009 was conducted.