FIGURE S-1 Influenza and pneumonia mortality by age, United States. Influenza and pneumonia specific mortality by age, including an average of the inter-pandemic years 1911–1915 (dashed line) and the pandemic year 1918 (solid line). Specific death rate is per 100,000 of the population in each age division.

SOURCES: Grove and Hetzel (1968); Linder and Grove (1943); U.S. Department of Commerce (1976).

Clues to Lethality and Adaptation

It remains unclear why the 1918 influenza virus was so deadly to otherwise healthy young adults. Historical data suggest its virulence was due in part to its novelty to people under the age of 30, who were not exposed to similar viral antigens during the apparent pandemics of the mid-and late 19th century. Children between the ages of 5 and 10 years were diagnosed with flu at higher than average rates, yet had the lowest mortality rates of any age group; that outcome may reflect a weak T-cell response to the virus, which is known to spare this age group from mortality due to measles (Taubenberger, 2004). By contrast, young adults may have mounted an unusual—and deadly—immune response to the 1918 virus. This possibility is supported by death records from Kentucky which, when analyzed on a year-by-year basis, reveal a precipitous rise to a peak in flu deaths beginning at age 17 and ending with a more gradual drop beginning around age 30 (Taubenberger, 2004). Researchers have also found epidemiological evidence that in the United States, people infected with tuberculosis (TB)—a relatively common infection in 1918, particularly among young males—

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