Influenza activity in Saint Joseph, Missouri 1910-1923: Evidence for an early wave of the 1918 pandemic.

: While the 1918/1919 H1N1 influenza pandemic is widely recognized as a "worst-case scenario" for the emergence of a new influenza strain, relatively little is known about the origin of the responsible virus and its pattern of spread. Most studies of this virus in the United States rely on temporally and spatially aggregated data. Location-specific studies of the impact of the 1918 pandemic strain in the United States have been confined primarily to large cities on the East Coast or West Coast. In this study, data on pneumonia and influenza fatalities from 1910-1923 have been extracted from death certificates for Saint Joseph, Missouri, a typical mid-sized city in the central United States. An increase in pneumonia and influenza mortality was noted starting in the 1915/1916 influenza season. Initially, increased mortality was observed in infants and the elderly. In February 1918, an age-shift typical of pandemic strains of virus was seen, as the burden of mortality shifted to young adults, a characteristic of the 1918 pandemic virus. These results provide one of the first confirmations of the existence of a "herald wave" of influenza activity in the United States prior to the recognized start of the H1N1 pandemic in Spring 1918. This study is one of very few that measures the impact of 1918/1919 influenza in a particular location in the central United States.