The 1918–1920 flu pandemic, known as the Spanish flu, was characterized by multiple waves of infection, each with distinct characteristics and varying levels of lethality. While the pandemic infected approximately 500 million people worldwide, its impact was not uniform, with the second wave proving to be significantly more deadly than the first. Understanding these waves is crucial to grasping the full scope of this historic global health crisis,
which ultimately caused the deaths of more than 50 million people worldwide.
The Relatively Mild First Wave
The first wave of the Spanish flu pandemic occurred during the first quarter of 1918 and was generally considered relatively mild. During this initial period, mortality rates were not significantly above normal levels. For instance, in the United States, approximately 75,000 flu-related deaths were reported in the first six months of 1918, a figure comparable to the 63,000 deaths during the same period in 1915. In Madrid, Spain, fewer than 1,000 people died from influenza between May and June 1918. There were no reported quarantines during this wave, and it primarily caused significant disruption to military operations during World War I, with a large proportion of French, British, and German soldiers falling ill.
The disease spread rapidly during this first wave, moving from Camp Funston in Kansas, a major training ground for American Expeditionary Forces troops, to other U.S. Army camps and then to Europe. By April 1918, it had become an epidemic in the Midwest, East Coast, and French ports, reaching the Western Front by mid-April. It quickly spread across France, Great Britain, Italy, and Spain, and by May, it had reached cities like Wrocław and Odessa. The release of Russian prisoners of war after the Treaty of Brest-Litovsk in March 1918 also contributed to its spread into Germany. By May, it had reached North Africa, India, and Japan, and by June, an outbreak was reported in China. After reaching Australia in July, this initial wave began to recede.
The Lethal Second Wave and its Aftermath
The second wave of the 1918 pandemic, which began in the fall of 1918, was far more deadly than the first. Unlike typical flu epidemics where the sick and elderly are most at risk, this wave had unusually high mortality for young adults. October 1918 recorded the highest fatality rate of the entire pandemic. In the United States, approximately 292,000 deaths were reported between September and December 1918, a stark contrast to the 26,000 deaths during the same period in 1915. The Netherlands reported over 40,000 deaths, and Bombay saw about 15,000 deaths in a population of 1.1 million. Historian David Arnold estimates at least 12 million deaths in India, about 5% of its population, making the 1918 flu pandemic particularly devastating there.
The severity of this second wave has been partly attributed to World War I. In civilian life, natural selection tends to favor milder strains, as severely ill individuals stay home, while those mildly ill continue their activities, spreading the less virulent strain. However, in the trenches of the war, this dynamic was reversed. Soldiers with mild strains remained on the front lines, while the severely ill were transported on crowded trains to packed field hospitals, inadvertently spreading the deadlier virus. This environment, combined with malnourishment, overcrowding, and poor hygiene, promoted bacterial superinfection, which killed most victims after a prolonged illness. The fact that many who recovered from first-wave infections became immune indicated it was the same strain of flu, but its increased lethality in the second wave was a critical factor in the pandemic's overall devastation.
Subsequent Waves and the Pandemic's End
Pandemic activity persisted into 1919 in many places, possibly due to winter conditions in the Northern Hemisphere, which is a typical time for influenza activity. However, the pandemic continued largely independent of region and climate. In the U.S., cases began to rise again in late November 1918, with a "recrudescence of the disease" reported in early December. This resurgence varied across the country, potentially influenced by differing restrictions. Michigan, for example, experienced a swift resurgence that peaked in December, possibly after the lifting of a ban on public gatherings. Pandemic interventions, such as bans on public gatherings and school closures, were reimposed in many areas to suppress the spread.
Between October 1919 and January 1920, 780,000 cases were reported across the U.S., with at least 20,000 deaths, indicating a severity three times greater than the corresponding period in 1918–1919, though the disease was generally regarded as milder but more infectious. Japan also saw a resurgence in December, peaking in January, leading to renewed pandemic measures and recommendations for mask use. A significant third wave developed in England and Wales by mid-February 1919, peaking in early March, and France and the Netherlands experienced similar peaks. By mid-1920, the pandemic was largely considered "over" by the public and governments, though some areas, like Chile and Peru, experienced further, albeit milder, waves into 1921. The winter of 1921–1922 marked the first major reappearance of seasonal influenza in the Northern Hemisphere since the main pandemic, affecting areas like northwestern Europe and the U.S.
