The impact of influenza epidemics on mortality: introducing a severity index

A cyclical regression model was applied to weekly national vital statistics from 1972-1992 to estimate excesses in pneumonia and influenza mortality and all-cause mortality for each influenza season. Each season was categorized on the basis of increments of 2000 pneumonia and influenza excess deaths, and each of these severity categories was correlated with a range of all-cause excess mortality. Over this period, influenza epidemics accounted for a total of 426,000 deaths in the US, many times more than those associated with recent pandemics. The index was useful for categorizing severity and provided improved seasonal estimates of the number of influenza-related deaths.