“Herd immunity” recently made a controversial appearance in the context of the current COVID-19 pandemic. What does the phrase mean, where did it come from, and how helpful is it today?
As of March 2020, the OED defines it as, “resistance to the spread of a contagious disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, typically as a result of having been vaccinated against it”.
The earliest use of the phrase can be traced to a 1917 report from the US Bureau of Animal Industry that dealt with a cattle infection causing death of unborn calves. A cow that had aborted was likely to become immune, and calves born and raised in such an affected herd were tolerant to the disease. The authors concluded that “a herd immunity seems to have developed as the result of both keeping the aborting cows and raising the calves”.
However, the senior author, Dr Adolph Eichhorn, Chief of the Pathological Division, made no reference to herd immunity in a monograph to which he contributed a major section on biological therapeutics just two years later. His biologically apt coinage does seem to have been picked up in US agricultural circles, but it was not universally adopted, with “immunity of the herd” being used instead.
The concept of herd immunity next appeared in British bacteriologist William Topley’s epidemiological studies of bacterial infection, which examined the resistance of a population of mice after immunising animals with suspensions of bacteria. He used “herd-resistance” to describe the natural resistance of individuals within a population. And he discussed the implications of his work with the “mouse herd” for the “human herd”.
The human herd entered this experimental realm at about the same time. In 1922, Surgeon-Commander Sheldon Dudley studied a diphtheria epidemic at Greenwich Hospital School. He found that the longer boys had been resident the greater the proportion who were immune, and that increases in immunity correlated with each outbreak. He extended such studies to other infectious diseases and used herd immunity to explain his findings.
In 1928, all boys in the school were actively immunised against diphtheria. The most senior became immune (Schick-test negative) twice as quickly as the most junior, suggesting prior exposure to the disease (see Figure). These results paralleled earlier work in animals, except for the fact that “a herd of human boys were used in lieu of the guinea-pigs”.
Dudley was unapologetic for using the prefix herd to denote the properties of a community, pointing out that psychologists had earlier popularized the phrase “herd instinct”. Besides, on evolutionary grounds, there was “little fundamental difference between a herd of deer, a herd of swine, and a herd of Homo sapiens”.
Notions of herd immunity have become more sophisticated in recent decades owing to the increased importance of vaccination. Today’s NHS website defines the benefits thus: “If enough people are vaccinated, it’s harder for the disease to spread to those people who cannot have vaccines. For example, people who are ill or have a weakened immune system”.
The reader is also directed to more information and an animation on the website of the Oxford Vaccine Group’s Vaccine Knowledge Project . This site suggests that a better name for herd immunity is “herd protection” because it helps to protect those especially vulnerable to infectious diseases. “Community immunity” appears as an alternative.
Conveying the value of herd protection or community immunity to the public will be critical in successful vaccination against COVID-19. One must worry that the lazy use of a century-old phraseology rooted in the farm, mouse lab and human guinea-pigs, as well as a contemporary profusion of alternative terms, may prove more of a hindrance than a help.
Words by Edward Wawrzynczak
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