Between 50 and 100 million unthinkable people around the world died from the 1918-1919 influenza pandemic, commonly known as the “Spanish flu”. It was the deadliest global pandemic since the Black Death, and rare among influenza viruses to have struck young and healthy people, often within days of the onset of symptoms. In the United States, the 1918 influenza pandemic reduced the average life expectancy by 12 years.
What’s even more remarkable about the 1918 flu, infectious disease experts say, is that it never really went away. After infecting around 500 million people worldwide in 1918 and 1919 (a third of the world’s population), the H1N1 strain that caused the Spanish flu has faded into the background and has remained the regular seasonal flu.
But every now and then the direct descendants of the 1918 flu combined with bird flu or swine flu to create powerful new pandemic strains, which is exactly what happened in 1957, 1968, and 2009. These latest influenza outbreaks, all created in part by the 1918 virus, claimed millions of additional lives, earning the 1918 flu the heinous title of “mother of all pandemics.”
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Deadly virus hit in three waves
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Jeffrey Taubenberger was part of the pioneering scientific team that isolated and sequenced the influenza virus genome from 1918 to the late 1990s. The painstaking process involved extracting viral RNA from autopsied lung samples taken from of American soldiers who died of the 1918 flu, plus a diseased lung kept in Alaskan permafrost for nearly 100 years.
Now head of the Viral Pathogenesis and Evolution section of the National Institutes of Health (NIH), Taubenberger explains that genetic analyzes of the 1918 flu indicate that it started out as avian flu and represented a whole new viral strain when it made the leap to humans. shortly before 1918. Laboratory tests of the reconstructed 1918 virus show that in its original form, the new encoded proteins of the virus made it 100 times more deadly in mice than the current seasonal flu.
The 1918 pandemic struck in three distinct waves over a 12-month period. It first appeared in the spring of 1918 in North America and Europe, largely in the trenches of World War I, then reemerged in its deadliest form in the fall of 1918, killing tens of millions people around the world from September to November. The last wave swept across Australia, the United States and Europe in late winter and spring 1919.
But did the 1918 flu simply “disappear” after this third wave? Absolutely not, says Taubenberger.
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The virus turns into the seasonal flu
A man receives a shave from a barber wearing a mask during the ongoing influenza pandemic, Chicago, Illinois, 1918.
Chicago Sun-Times / Chicago Daily News Collection / Chicago History Museum / Getty Images
Since the whole world had been exposed to the virus and therefore developed natural immunity to it, the 1918 strain began to mutate and evolve in a process called “antigenic drift.” Slightly modified versions of the 1918 flu reappeared during the winters of 1919-1920 and 1920-1921, but they were much less fatal and almost indistinguishable from the seasonal flu.
“The 1918 flu definitely lost its real virulence in the early 1920s,” says Taubenberger.
But what’s really amazing, according to genetic analyzes, is that the same new strain of influenza first introduced in 1918 appears to be the direct ancestor of every seasonal and pandemic influenza that we have had in the past century. .
“You can still find the genetic traces of the 1918 virus in the seasonal influenza that circulates today,” says Taubenberger. “Every human infection with influenza A over the past 102 years has been derived from the introduction of influenza in 1918.”
Welcome to the era of the pandemic
The 1918 influenza pandemic was by far the deadliest influenza epidemic of the 20th and 21st centuries to date, but it was not the only one to be labeled a pandemic. Even with the advent of the first seasonal influenza vaccines after World War II, the influenza virus has been shown to be capable of some unexpected and deadly genetic tricks.
During a normal flu season, vaccine specialists can track the most active viral strains and produce a vaccine that protects against changes in the human flu virus from year to year. But every now and then viral genes from the animal kingdom get into the mix.
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“If an animal is infected with two different influenza viruses at the same time,” says Taubenberger, “maybe one virus from a bird and another from a human, these genes can mix together to create a whole new virus that never existed before. “
This is what happened in 1957 when the 1918 flu, which is an H1N1 virus, swapped genes with another bird flu, giving us the H2N2 pandemic, which claimed a million lives worldwide. It happened again in 1968 with the creation of the so-called “Hong Kong Flu”, an H3N2 virus that killed another million people.
The so-called “swine flu” pandemic of 2009 has an even deeper history. When humans were infected with the pandemic flu of 1918, which was originally bird flu, we also passed it on to pigs.
“A branch of the 1918 flu permanently adapted to pigs and became swine flu which was seen in pigs in the United States every year after 1918 and spread around the world,” explains Taubenberger.
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In 2009, a strain of swine flu swapped genes with both human and bird flu to create a new variety of H1N1 that “looked more like 1918 than it had been seen in a long time,” says Taubenberger. About 300,000 people died from the 2009 influenza pandemic.
In total, if 50 to 100 million people died in the pandemics of 1918 and 1919, and tens of millions more died in the next century from seasonal influenza and pandemic epidemics, then all of those deaths can be attributed to the unique and accidental human emergence of the highly successful and stubborn 1918 virus.
“We are still living in what I would call the ‘1918 pandemic era’ 102 years later,” Taubenberger says, “and I don’t know how long that will last.”
READ MORE: See all pandemic coverage here.
