The Spanish flu lasted roughly two years, from March 1918 through the summer of 1919 in its three main waves. A less well-known fourth wave struck parts of the United States in early 1920, stretching the pandemic’s full impact closer to two and a half years. During that time, at least 50 million people died worldwide and about 675,000 in the United States alone.
Where and When It Started
The earliest cases of unusual severity appeared in January and February of 1918 in Haskell County, a rural area in southwestern Kansas. A local physician reported flu cases that were far more aggressive than the typical seasonal illness. Young men from the county soon left for military service at Camp Funston, a massive training camp in eastern Kansas housing over 50,000 soldiers. By early March, hundreds of soldiers were sick and many had died. From there, troop movements during World War I carried the virus across the country and then overseas.
Three Waves in 18 Months
The pandemic moved through the population in three distinct surges, each with its own character.
The first wave hit in the spring of 1918. It spread quickly through military camps and cities but was relatively mild compared to what followed. Many people got sick, but the death rate during this initial surge was not dramatically higher than a bad seasonal flu. This wave faded over the summer months.
The second wave, beginning in the fall of 1918, was catastrophic. This was the phase responsible for most of the pandemic’s deaths in the United States. October 1918 was the single deadliest month: an estimated 195,000 Americans died from the flu in those 31 days alone. The virus had mutated into something far more lethal, killing otherwise healthy young adults at a rate no one had seen before. Cities that had relaxed their guard over the summer were overwhelmed within weeks.
A third wave rolled through during the winter and spring of 1919, adding significantly to the death toll before subsiding by summer. While less explosive than the fall peak, it was still far deadlier than ordinary flu seasons and extended the crisis for months longer than many communities expected.
The Overlooked Fourth Wave in 1920
Most histories describe the pandemic as a three-wave event ending in mid-1919, but a fourth wave swept through several parts of the United States in early 1920. New York City, Chicago, and the U.S. territory of Hawaii all experienced rapid outbreaks. Some communities that had escaped the worst of the earlier waves were hit hardest this time around. The village of Nenana in Alaska, which had avoided the first three surges, reported a nearly 100 percent infection rate during the fourth wave and lost close to 10 percent of its population. Alaska Native communities in rural villages were particularly devastated.
This fourth wave is a reminder that the pandemic’s timeline wasn’t clean or uniform. Different regions experienced the worst of it at different points, and isolated communities sometimes faced their deadliest outbreaks long after the virus had receded elsewhere.
How the Pandemic Actually Ended
The Spanish flu didn’t vanish overnight. It wound down gradually as enough of the global population developed immunity through infection. But the virus itself didn’t disappear. The 1918 strain was an H1N1 influenza virus, and descendants of it continued circulating for years. By around 1922, the original pandemic strain appears to have been displaced in humans by a reassortant virus, one that had swapped in a slightly different version of its key surface protein through mixing with another flu strain already circulating at the time. The result was a less dangerous seasonal H1N1 virus that kept spreading but no longer caused mass death.
In practical terms, the transition from pandemic to endemic happened over several years. The acute emergency, with its mass graves and shuttered businesses, lasted from mid-1918 through early 1920 in most places. But elevated flu deaths continued beyond that as the virus slowly settled into a seasonal pattern. The descendants of the 1918 virus, in various mutated and reassorted forms, circulated in humans for decades afterward.
Why the Timeline Varied So Much by Location
One reason the question “how long did it last” is hard to pin down is that the pandemic hit different places at very different times. A city like Philadelphia experienced its worst week in October 1918. Rural Alaskan villages didn’t see their worst outbreak until early 1920. Island nations and remote communities sometimes delayed the virus’s arrival by months or years through natural isolation, only to face devastating outbreaks when the virus finally reached them.
The global consensus places the core pandemic at March 1918 through summer 1919, a span of about 15 months across three waves. Including the fourth wave extends the active pandemic period to roughly two years. And the full transition from pandemic virus to seasonal flu took until about 1922, making the total period of abnormally high mortality closer to four years from the first known cases.