Most victims were healthy young adults, in contrast to most influenza outbreaks, which predominantly affect juvenile, elderly, or weakened patients.
An unusual feature of this pandemic was that it mostly killed young adults, with 99% of pandemic influenza deaths occurring in people under 65, and more than half in young adults 20 to 40 years old.[48] This is noteworthy, since influenza is normally most deadly to weak individuals, such as infants (under age two), the very old (over age 70), and the immunocompromised. In 1918, older adults may have had partial protection caused by exposure to the previous Russian flu pandemic of 1889.[49] According to historian John M. Barry, the most vulnerable of all – "those most likely, of the most likely", to die – were pregnant women. He reported that in thirteen studies of hospitalized women in the pandemic, the death rate ranged from 23% to 71%. Of the pregnant women who survived childbirth, over one-quarter (26%) lost the child.[50]
Another oddity was that the outbreak was widespread in the summer and autumn (in the Northern Hemisphere); influenza is usually worse in winter.[51]
Modern analysis has shown the virus to be particularly deadly because it triggers a cytokine storm, which ravages the stronger immune system of young adults.[9]
The second wave of the 1918 pandemic was much deadlier than the first. The first wave had resembled typical flu epidemics; those most at risk were the sick and elderly, while younger, healthier people recovered easily. But in August, when the second wave began in France, Sierra Leone and the United States,[52] the virus had mutated to a much deadlier form. This has been attributed to the circumstances of the First World War.[53]
In civilian life, natural selection favours a mild strain. Those who get very ill stay home, and those mildly ill continue with their lives, preferentially spreading the mild strain. In the trenches, natural selection was reversed. Soldiers with a mild strain stayed where they were, while the severely ill were sent on crowded trains to crowded field hospitals, spreading the deadlier virus. The second wave began and the flu quickly spread around the world again.[54]
It was the same flu in that most of those who recovered from first-wave infections were immune, but it was now far more deadly; the most vulnerable people were those like the soldiers in the trenches—young, otherwise-healthy adults.[55] Consequently, during modern pandemics health officials pay attention when the virus reaches places with social upheaval (looking for deadlier strains of the virus).[54]
This effect was most dramatically illustrated in Copenhagen, which escaped with a combined mortality rate of just 0.29% (0.02% in the first wave and 0.27% in the second wave) because of exposure to the less-lethal first wave.[56]
