How did the 1918 influenza pandemic spread and end?

The 1918 pandemic was caused by an H1N1 virus with genes of avian origin. It spread worldwide along the routes of troop movements, railways and shipping lines during the First World War, in a series of waves between 1918 and 1920. It faded because so many people had been infected and gained immunity that the virus had too few susceptible hosts. Its place of origin is not agreed upon. [1] [2]

In the United States the virus was first identified in military personnel in spring 1918. On March 4, 1918, military doctors formally identified it in Army cook Albert Gitchell at Camp Funston, Kansas. Within weeks more than 1,000 soldiers there were ill, and the virus was reported at most Army training camps. Soldiers then carried it to the East Coast and infected civilians at railroad hubs and at embarkation and debarkation ports. [3] [1]

By the end of April 1918 influenza was spreading along the Western Front. North Africa, India and Japan reported widespread infections in May, China in June and Australia by July. A Journal of Global History article identifies three kinds of network that carried the disease: military, colonial and trade networks. As troops went home at the war's end in late 1918, they took the disease with them. The same article notes that trench warfare provided favourable conditions for transmission. [3] [4]

The pandemic is generally described as a mild 'herald' wave in spring and early summer 1918, a deadly autumn wave, and one or more later waves in 1919 or 1920. The waves were unusually synchronized around the world. The autumn wave began in August in France and Spain and reached Russia in November, while North America experienced it between August and December 1918. A third wave hit Australia and parts of Europe hard in early 1919, although mortality fell markedly in American cities. A fourth wave struck countries including Canada, Japan and Taiwan, and parts of the US, in February 1920. The authors caution that this standard picture is disproportionately based on American and western European experience. [4] [3]

One source says the first wave apparently gave some immunizing protection against the more virulent variants that followed. [5]

Historians at the University of Michigan's Center for the History of Medicine say the pandemic ended because the virus circulated widely enough that too few susceptible people remained for it to sustain a pandemic. By the end, about a third of the world's population had caught it. There was no dramatic declaration; by the middle of 1920 the deadly strain had faded in many places. [2]

The Journal of Global History article describes a later 'settling-in' to ordinary seasonal patterns, with later waves in 1920 and 1921 occurring during flu seasons. The Census Bureau notes that the virus may also have mutated to a more benign form, and that influenza deaths in 1921 were comparable to pre-pandemic levels. Howard Markel observes that flu tends to go quiet when cold weather recedes, but that no one knows why. [4] [3] [2]