The 1918 pandemic killed young adults at an alarming rate
November 30, 2006
One of the most noted traits of the 1918 pandemic was the unusual age distribution of its victims. We should plan for the possibility that future pandemics could have the same effect.
Normally, if you draw a chart showing flu mortality - with the X-axis representing increasing ages - the chart shows peaks in mortality for young children and the elderly, the left and right ends of the age spectrum. The chart line is "U-shaped." That's the normal distribution.
Flu pandemics aren't normal. (Isn't that THE core message in all discussions about pandemics?)
If you look at a mortality chart for the 1918 pandemic, there are three peaks - one for the very young, one for the elderly, and one for young adults. The three peaks look like a "W"; this is called a "W-shaped" mortality curve.
The U.S. National Museum of Health and Medicine has an old chart (see right) which shows this in an interesting comparison. The chart plots the annual mortality rate per 100,000 people. The top curve shows the average number of deaths from influenza and pneumonia in Boston during September-November for the years 1912-16. That's the normal baseline for comparison. Note the curve's U-shape.
The middle curve shows the influenza-pneumonia mortality for September-October 1918, a two-month segment of the pandemic. Note the unusual middle peak, centered on the 20-29 age group, making a W-shape curve.
The bottom curve shows what a dramatic reversal this was from normal influenza-pneumonia mortality. This curve plots the ratio of the top two curves. While the 0-9 age group and the "elderly"* 40+ age group died at a rate 5-to-10 times greater than normal, the mortality rate for the 20-29 age group was 175 times greater than normal! The 10-19 age group was about 100-fold the normal rate and the 30-39 age group was about 60-fold higher.
Therefore, the 1918 flu pandemic reversed which age groups were at greatest risk. The same shift is seen in the milder pandemics of 1957 and 1968.
To my knowledge, we don't know why healthy young adults were disproportionately killed. One theory is that the young adults' immune systems were stronger than those of the very young and elderly. These immune systems responded so intensely to the virus' attack that the victims were killed in the process.
FOOTNOTE:
* "Elderly" is a relative concept. Today it seems odd to refer to the "40+" age group as "elderly." But life expectancy for white males born then was 56 years.