Cases and deaths during influenza pandemics in the United States

Objective: To assess how numbers and age distributions of cases and deaths during an influenza pandemic in the United States would potentially vary from those during the 1918-1919 pandemic, given the same virulence of the pandemic strain. Methods: Influenza cases and deaths in two referent populations (U.S. residents in 1917 and 2006) were calculated using clinical case rates from three pandemics (1918-like, 1957-like, and 1968-like) and case fatality rates from the 1918-1919 pandemic. Results: Across pandemic scenarios, overall case ("attack") rates ranged from 24.7% to 34.2%, and overall death rates ranged from 4.4 to 6.7 per 1000. In both referent populations, total cases and deaths were significantly higher when using 1957-like and 1968-like, compared to 1918-like, case rates. Under all pandemic scenarios, the most deaths occurred among those aged 25 to 29 years. However, in the 2006 referent population, there were large numbers and high proportions of deaths in middle-aged and elderly adults (unlike during the 1918-1919 pandemic). Conclusions: Numbers and distributions of cases and deaths during influenza pandemics depend on numbers of individuals, clinical case rates, and case fatality rates in relation to age. During a future pandemic in the United States, influenza deaths will likely not be as sharply focused in young adults as in 1918-1919 (even if case fatality rates are similar) because of larger proportions of middle-aged and elderly adults and potentially higher case rates among adults older than 30.