Trends in recorded influenza mortality: United States, 1900-2004

I sought to describe trends in historical influenza mortality data in the United States since 1900 and compare pandemic with nonpandemic influenza seasons.

I compiled a database of monthly influenza-classed death rates from official US mortality tables for the years 1900 to 2004 (1905-1909 excluded), from which I calculated adjusted influenza season (July 1-June 30) mortality rates.

An overall and substantial decline in influenza-classed mortality was observed during the 20th century, from an average seasonal rate of 10.2 deaths per 100 000 population in the 1940s to 0.44 per 100 000 by the 1990s [corrected] . The 1918-1919 pandemic stands out as an exceptional outlier. The 1957-1958 and 1968-1969 influenza pandemic seasons, by contrast, displayed substantial overlap in both degree of mortality and timing compared with nonpandemic seasons.

The considerable similarity in mortality seen in pandemic and non-pandemic influenza seasons challenges common beliefs about the severity of pandemic influenza. The historical decline in influenza-classed mortality rates suggests that public health and ecological factors may play a role in influenza mortality risk. Nevertheless, the actual number of influenza-attributable deaths remains in doubt.