As many countries around the world recognised the magnitude of the COVID-19 pandemic in March, 2020, some seemed to put their faith in herd immunity. UK pandemic adviser Graham Medley, for example, said that “We are going to have to generate what we call herd immunity”, which would require “a nice big epidemic”. When the idea received furious criticism, British officials denied that herd immunity had ever been part of their plan. A run at herd immunity in Sweden prompted mathematician Marcus Carlsson to object: “we are being herded like a flock of sheep toward disaster”. In August, WHO’s Michael Ryan warned journalists “we are nowhere close to the levels of immunity required to stop this disease transmitting. We need to focus on what we can actually do now to suppress transmission and not live in hope of herd immunity being our salvation.” That did not end the debate. In late August sources revealed that the White House might be pondering a policy of herd immunity. Officials issued a prompt denial. The appeal of herd immunity is easy to understand: if it is reached, an epidemic ends. But the illness and death such an approach would require have prompted a strong backlash. The language of herd immunity is part of the problem. A herd usually describes domesticated animals, especially livestock. Herd animals like cows, goats, or sheep are sacrificed for human consumption. Few humans want to be part of that kind of herd.
How did herd immunity enter the language of public health? The phrase seems to have first appeared in the work of American livestock veterinarians concerned about “contagious abortion”—epidemics of spontaneous miscarriage—in cattle and sheep. By the 1910s, it had become the leading contagious threat to cattle in the USA. Farmers destroyed or sold affected cows. Kansas veterinarian George Potter realised that this was the wrong approach. Writing with Adolph Eichhorn in 1916 in the Journal of the American Veterinary Medical Association, he envisioned “herd immunity”. As he wrote in 1918, “Abortion disease may be likened to a fire, which, if new fuel is not constantly added, soon dies down. Herd immunity is developed, therefore, by retaining the immune cows, raising the calves, and avoiding the introduction of foreign cattle.”
Potter’s concept reached the UK in 1917 and 1920 in summaries in Veterinary Review and Scottish Agriculture. It arrived at a crucial moment. Armies and navies struggled against infections throughout World War 1. Medical professionals worked to identify and treat pathogens, and also to understand their population ecology. How did pathogen virulence and population resistance drive the rise and fall of epidemic waves? In The Lancet in July, 1919, bacteriologist W W C Topley described experimental epidemics he created in groups of mice. Unless there was a steady influx of susceptible mice, the rising prevalence of immune individuals would end an epidemic. In a 1923 article in the Journal of Hygiene, he and G S Wilson described this phenomenon as “herd immunity”.
The idea moved into medicine. In 1922, Topley suggested a parallel between outbreaks in mice and children: “Such a likeness would seem to exist in the case of epidemic diseases affecting children of school age.” He also wondered whether measures already “in vogue in dealing with epidemics among live-stock, where methods of segregation are so much more easily enforced than among human populations”, might inform decisions about school closings amid epidemics.
Topley’s musings soon found their test. In 1923 Sheldon Dudley, professor of pathology at the Royal Naval Medical School, became aware of epidemics of diphtheria at…
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