What Was the 1918 Flu Pandemic? The Deadliest Year in Modern History
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In about two years an influenza virus infected perhaps a third of the world's population and killed somewhere between fifty and a hundred million people, which is more than both world wars combined and more in a single year than the Black Death killed in a century. It did not start in Spain. It killed healthy young adults in preference to the old. And for eighty years nobody knew what had made it so lethal, because the virus itself had vanished before anyone could isolate it.
Where it came from and what it was called
The name is a wartime artefact. The pandemic arrived in the middle of the First World War, and the combatant countries censored reports of illness among their troops and populations to protect morale. Spain was neutral and its press was not censored, so Spanish newspapers reported the outbreak freely, including the king's own illness, and the rest of Europe concluded that Spain was where it was happening. Spaniards called it the French flu. The actual origin is still debated, with the three main candidates being an American army camp in Kansas, where an outbreak was recorded in March 1918, a British army camp at Etaples in northern France, where a respiratory illness with unusual features was described in 1916 and 1917, and a site in northern China from which labourers travelled to Europe. The virus was an H1N1 influenza A of avian origin, and genetic work suggests its ancestor entered humans a few years before the pandemic.
The three waves
The pattern was unusual and is central to understanding what happened:
- •A first wave in spring 1918 was widespread and comparatively mild, and in many places was not recognised as anything unusual
- •A second wave from August to December 1918 was catastrophic, and killed the great majority of the pandemic's victims in about thirteen weeks
- •A third wave in early 1919 was severe but less so, with a further smaller wave in some places in 1920
- •The second wave's severity is usually attributed to a mutation, and to wartime conditions that selected for it, since a soldier too ill to move was still transported by train to a crowded hospital, which rewards a virus that incapacitates rather than one that lets its host walk around
- •Troop movements at the war's end distributed it globally within weeks, reaching remote communities with no prior exposure, where mortality in some Alaskan and Pacific island villages exceeded a quarter of the population
Who it killed
Influenza normally kills the very young and the very old, producing a mortality curve shaped like a U. This one produced a W, with an enormous additional peak among people aged roughly twenty to forty, who are ordinarily the least at risk. Two explanations are offered and both may hold. The first is an overwhelming immune response, in which a vigorous young immune system floods the lungs with fluid and inflammatory cells, drowning the patient in their own response, which is consistent with the rapid deterioration described in case notes. The second is that people born before around 1890 had been exposed to an earlier influenza virus that conferred partial cross-protection, while those born after had not, which would explain the age band precisely. Most deaths were not from the virus directly but from secondary bacterial pneumonia, in an era with no antibiotics, which is why the case fatality rate of a similar virus today would be far lower.
What was done about it
There were no antivirals, no vaccine and no understanding of what caused it, since the influenza virus was not identified until 1933 and many physicians believed the disease was bacterial. What remained were the measures now called non-pharmaceutical: isolation, quarantine, closing schools, theatres and churches, banning public gatherings, staggering business hours, and masks. Their effectiveness has been analysed retrospectively in detail, and the results are clear. Cities that acted early and maintained measures had substantially lower peak death rates, and the comparison between Philadelphia, which allowed a war bond parade with two hundred thousand attendees on 28 September 1918 and saw deaths explode within days, and St Louis, which closed public gathering places promptly, is the standard teaching example. A 2007 analysis of forty-three American cities found that early and sustained intervention correlated with lower mortality, and a related study found that cities which intervened more aggressively did not suffer worse economically, and in several cases recovered faster.
Recovering the virus
The pathogen itself was retrieved decades later through an extraordinary piece of work. Johan Hultin, a Swedish microbiologist, attempted in 1951 to recover tissue from victims buried in Alaskan permafrost and failed to obtain viable material. In 1997 he returned at the age of seventy-two, funding the trip himself, and obtained lung tissue from a woman whose body fat had helped preserve her organs. That material, combined with samples from preserved autopsy specimens held by the American armed forces, allowed Jeffery Taubenberger's team to sequence the complete genome, published in 2005, and then to reconstruct the live virus under maximum containment. Experiments with the reconstructed virus showed extreme virulence in animal models and identified specific genetic features responsible, which is medically valuable and was also contentious, since publishing a complete recipe for a virus that killed tens of millions raised obvious questions that the biosecurity community is still arguing about.
The takeaway
An H1N1 influenza of avian origin infected around a third of humanity in 1918 and 1919 and killed between fifty and a hundred million, with a mild first wave followed by a catastrophic second in which most deaths occurred in about thirteen weeks. It was named for Spain only because Spanish newspapers were uncensored. It killed young adults disproportionately, probably through immune overreaction and the absence of cross-protection, and most deaths came from secondary bacterial pneumonia. The genome was recovered from a body in Alaskan permafrost and published in 2005.