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Patterns of fatality

The pandemic mostly killed young adults. In 1918–1919, 99% of pandemic influenza deaths in the U.S. occurred in people under 65, and nearly half of deaths were in young adults 20 to 40 years old. In 1920, the mortality rate among people under 65 had decreased sixfold to half the mortality rate of people over 65, but 92% of deaths still occurred in people under 65. This is unusual since influenza is typically most deadly to weak individuals, such as infants under age two, adults over age 70, and the immunocompromised. In 1918, older adults may have had partial protection caused by exposure to the 1889–1890 flu pandemic, known as the "Russian flu". 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. Another oddity was that the outbreak was widespread in the summer and autumn (in the Northern Hemisphere); influenza is usually worse in winter.

There were also geographic patterns to the disease's fatality. Some parts of Asia had 30 times higher death rates than some parts of Europe, and generally, Africa and Asia had higher rates, while Europe and North America had lower ones. There was also great variation within continents, with three times higher mortality in Hungary and Spain compared to Denmark, two to three times higher chance of death in Sub-Saharan Africa compared to North Africa, and possibly up to ten times higher rates between the extremes of Asia. Cities were affected worse than rural areas. There were also differences between cities, which might have reflected exposure to the milder first wave giving immunity, as well as the introduction of social distancing measures.

Another major pattern was the differences between social classes. In Oslo, death rates were inversely correlated with apartment size, as the poorer people living in smaller apartments died at a higher rate. Social status was also reflected in the higher mortality among immigrant communities, with Italian Americans, a recently arrived group at the time, were nearly twice as likely to die compared to the average Americans. These disparities reflected worse diets, crowded living conditions, and problems accessing healthcare. Paradoxically, however, African Americans were relatively spared by the pandemic.

More men than women were killed by the flu, as they were more likely to go out and be exposed, while women would tend to stay at home. For the same reason men also were more likely to have pre-existing tuberculosis, which severely worsened the chances of recovery. However, in India the opposite was true, potentially because Indian women were neglected with poorer nutrition, and were expected to care for the sick.

A study conducted by He et al. (2011) used a mechanistic modeling approach to study the three waves of the 1918 influenza pandemic. They examined the factors that underlie variability in temporal patterns and their correlation to patterns of mortality and morbidity. Their analysis suggests that temporal variations in transmission rate provide the best explanation, and the variation in transmission required to generate these three waves is within biologically plausible values. Another study by He et al. (2013) used a simple epidemic model incorporating three factors to infer the cause of the three waves of the 1918 influenza pandemic. These factors were school opening and closing, temperature changes throughout the outbreak, and human behavioral changes in response to the outbreak. Their modeling results showed that all three factors are important, but human behavioral responses showed the most significant effects.