Influenza as a proportion of pneumonia mortality: United States, 1959-2009.

As causes of death, influenza and pneumonia are typically analyzed together. We quantify influenza's contribution to the combined pneumonia and influenza mortality time series for the United States, 1959-2009. A key statistic is I/(P + I), the proportion of pneumonia and influenza mortality accounted for by influenza. Year-to-year, I/(P + I) is highly variable and shows long-term decline. Extreme values of I/(P + I) are associated with extreme P + I death rates and vice versa, but I/(P + I) is a weak predictor of P + I mortality overall. Prominence of influenza in the medical news is associated with high I/(P + I). Influenza and pneumonia should be analyzed as a combined cause.