Scientists have announced that they may have solved one of history’s biggest biomedical mysteries—why the deadly 1918 “Spanish flu” pandemic, which killed perhaps 50 million people worldwide, largely targeted healthy young adults.
The explanation turns out to be surprisingly simple: People born after 1889 were not exposed as kids to the kind of flu that struck in 1918, leaving them uniquely vulnerable.
Older people, meanwhile, had been exposed to flu strains more closely related to the 1918 flu, offering some immunity.
Simply put, the Spanish flu owed its ferocity to a switch in dominant influenza varieties that had occurred a generation earlier. (Related: “1918 Flu That Killed 50 Million Originated in China.”)
“All a matter of timing,” says virologist Vincent Racaniello of Columbia University in New York, who was not part of the study.
Researchers involved in the study looked at the evolutionary history of the components of the 1918 flu, which was built of genes from human and avian flu strains. They unraveled the history of dominant flu strains stretching back to 1830.
The evolutionary biologists found that a worldwide 1889 outbreak of the so-called Russian flu, the H3N8 flu virus, left a generation of children that had not been exposed to anything resembling the Spanish flu, which was an H1N1 strain.
(The H and N in the flu designation stand for proteins called hemagglutinin and neuraminidase, respectively).
The spread of a more closely related H1 flu variety after 1900 provided partial immunity to children born after that time. That closed the window of vulnerability.
“You have the most deadly flu pandemic in history essentially leaving the elderly, its most frequent victims, completely alone,” says biologist Michael Worobey of the University of Arizona in Tucson, who led the Proceedings of the National Academy of Sciences report.
Instead, people aged 18 to 29 died in droves during the outbreak, which killed about 1 in 200 of victims.
Experts have suggested that such a window of vulnerability partly explained the 1918 pandemic, Racaniello notes.
But the new study provides computational evidence that the 1918 flu’s precursor originated around 1907, he says, and explains how the window of vulnerability opened and closed for the disease.