The 1918 flu pandemic stands out in history as one of the world’s deadliest—it’s believed to have killed between 1.3 and 3 percent of the human population. But based on doctors’ reports from the time, the disease was peculiar for another reason: It had an unusually high death rate among young adults.
Historical accounts and later research suggested the 1918 flu targeted healthy people as well as those with underlying conditions. But based on how infectious diseases today behave—with most deaths occurring among the very young or elderly—this baffled scientists.
Now, a new study published Monday in the journal Proceedings of the National Academy of Sciences suggests the 1918 flu may have been more severe for frail or unhealthy people. An analysis of skeletal remains from the time period found that people whose bones showed signs of stress were more likely to die during the pandemic.
“This idea that the 1918 flu killed healthy young people is not supported by our findings,” Sharon DeWitte, a co-author of the study and biological anthropologist at the University of Colorado, Boulder, says in a statement from the university. “Instead, we found that this pandemic, like many others across history, disproportionately killed frail people.”
“It’s an important study,” Andrew Noymer, an epidemiologist at the University of California, Irvine, who did not contribute to the findings, tells Science’s Mitch Leslie. “This will help set the record straight about the 1918 flu.”
The influenza pandemic of 1918 and 1919 was the deadliest flu outbreak of the 20th century. It may have killed as many as 50 to 100 million people, and it infected one in three humans across the planet. Among cities and states that kept data in the United States, the most deaths occurred in young people ages 20 to 34, and pregnant people were the group most likely to die from the flu.
To test the idea that healthy and weaker people were equally vulnerable, the researchers turned to a collection of more than 3,000 human skeletons at the Cleveland Museum of Natural History. They examined the bones of 369 people who died before or during the flu outbreak and looked for lesions on shin bones to determine their health—these regions of damage would indicate the person had experienced trauma, infection, malnutrition or some other stressor that would have made them unhealthy, per the statement.
Peter Palese, a flu expert at the Icahn School of Medicine at Mount Sinai who was not involved in the study, tells the New York Times’ Gina Kolata that this method for identifying frailty “is quite legitimate.”
The researchers found that people whose bones showed active lesions were more likely to have died during the pandemic than healthy people whose lesions were healed.
However, Lone Simonsen, an epidemiologist at Roskilde University in Denmark who was not involved in the research, notes to Science that death rates were not higher among the elderly for the 1918 flu, which would be expected if the flu had targeted frailer people. “We don’t see deaths in the groups where frailty is more common,” Simonsen tells the publication.
While further research is needed, the study authors theorize that, like the Covid-19 pandemic, factors like socioeconomic status, institutional racism and access to healthcare could have had an impact on who faced the greatest risks from the flu, per the statement.
The takeaway from the study is that “we should never expect any nonaccidental cause of death to be indiscriminate,” DeWitte says to the New York Times.