Understanding How Spanish Flu Progressed and the Death Curve
Before and after 1918, most influenza pandemics developed in Asia and spread from there to the rest of the world. To the contrary, the Spanish flu pandemic spread more or less simultaneously in 3 distinct waves during a 12-month period from 1918–1919, in Europe, Asia, and North America (the first wave was best described in the United States in March 1918).
The first pandemic influenza wave appeared in the spring of 1918, followed in rapid succession by much more fatal second and third waves in the fall and winter of 1918–1919, respectively. The Spanish flu pandemic had another unique feature, the simultaneous (or nearly simultaneous) infection of humans and swine.
The curve of influenza deaths by age has, historically, for at least 150 years, been U-shaped, with death peaks in the very young and the very old, with a comparatively low number of deaths at all ages in between.
In contrast, age-specific death rates in the Spanish flu pandemic showed a distinct pattern that has not been documented before or since: a "W-shaped" curve, similar to the familiar U-shaped curve, but with the addition of a third (middle) distinct peak of deaths in young adults who were between 20 and 40 years of age.
Influenza and pneumonia death rates for those 15 to 34 years of age in 1918–1919, for example, were 20 times higher than in previous years. Overall, nearly half of the influenza-related deaths in the 1918 pandemic were in young adults who were 20 to 40 years of age, a phenomenon unique to that pandemic year.
The Spanish flu pandemic is also unique among influenza pandemics in that absolute risk of influenza death was higher in those who were younger than 65 years of age than in those who were greater than 65 years of age. People who were younger than 65 accounted for 99 percent of all excess influenza-related deaths in 1918–1919. In comparison, those who were older than 65 accounted for 36 percent of all excess influenza-related deaths in the 1957 H2N2 pandemic and 48 percent in the 1968 H3N2 pandemic.