Mischievous Immunity: How Intellectual History Can Be Relevant for Current Public Health Practice

Herd immunity has had a banner year. After decades consigned to the writings of mathematical modelers and vaccine advocates, it burst onto the public stage in March 2020 as the world woke to the threat of COVID-19. The concept first surfaced in London, United Kingdom, where senior science advisors wondered whether herd immunity could be safely achieved by allowing the infection of low-risk individuals.¹ After this initial cameo, herd immunity took two world tours. The first involved policy, with officials in Sweden, Brazil, and the United States considering the merits of widespread epidemics. This idea received its most vigorous backing, and its most strident rejection, from the Great Barrington Declaration in October.² The second tour involved people: throughout 2020 observers debated whether specific populations had achieved herd immunity. Early contenders included Corona, New York (68% had antibodies in July); Dharavi, in Mumbai, India (57% by July); and Manaus, Brazil (76% by October). COVID-19’s surges in spring 2021 cast doubt on those hopes.^{3 , 4} Despite growing evidence of the efficacy of vaccination, there is also growing skepticism about whether SARS-CoV-2 will ever succumb to herd immunity. Imperfect vaccines, vaccine hesitancy, and the evolution of variant strains might allow the pandemic to persist.^{5 , 6}

Heightened public interest has inspired scholarly investigations. Epidemiologists and clinicians have published reviews of the theory and practice of herd immunity.^{7 , 8} Others have explored its history.⁹ In “Of Mice and Schoolchildren: A Conceptual History of Herd Immunity” (p. 1473), David Robertson offers new insights. Digging deeply into writings of livestock veterinarians, he has found herd immunity’s earliest occurrence yet, in an 1894 article about immunity and hog nutrition by David Salmon, the first director of the US Department of Agriculture’s Bureau of Animal Husbandry. Although Salmon’s understanding of immunity was quite different from modern theories, he seemed to see herd immunity as the sum of the immunities of individuals in a herd.

As Robertson shows, herd immunity came to mean different things to different scientists. In the 1920s William Topley and Graham Wilson saw herd immunity not as the sum of individual immunities but as an emergent property of a herd. In the 1950s Jonas Salk wrote about the “herd effect,” the penumbra of immunity that extended beyond those who had been vaccinated. Herd immunity gained its current meanings with the work of Robert May and Roy Anderson in the 1980s. Herd immunity was often complex and multifaceted. Over the past year, however, public discourse has focused on a narrow question: what percentage of a population must be exposed or vaccinated to achieve herd immunity? Robertson’s history suggests that there will never be a single answer. Salmon and a century of subsequent theorists identified many factors that shape the immune dynamics of a population. Nor will it be easy to determine whether it is better during the COVID-19 pandemic to provide partial immunity to as many people as quickly as possible (the UK approach) or to provide full immunity to half as many (the US approach). There are too many unknowns (as I write this in May) to know.

Robertson’s essay also raises important questions for intellectual history. The increasing availability of digitized sources (some newly available because of COVID-19) makes it possible to search vast online collections and chart the origins of concepts. Historians have traced the emergence of “herd immunity,” “virgin soil epidemics,” and “risk factors.”¹⁰ Sometimes they strike gold, as with Robertson’s discovery of Salmon. But this approach leaves some mysteries unsolved. What happened to herd immunity between 1894 (Salmon’s work on pigs) and 1916 (Adolf Eichhorn and George Potter’s work on contagious abortion in cattle)? Potter and Eichhorn worked for the US Department of Agriculture. Did they know Salmon? Another US agricultural researcher, R. R. Birch, also used the phrase around this time in discussions of swine cholera. And how did the phrase jump the Atlantic? Major Greenwood, who studied swine fever, may have played a role, but a direct link has not yet been found. As more and more sources are digitized, future historians will likely find earlier occurrences. But some answers will require traditional methods (e.g., analyses of archives and personal papers) and detailed knowledge of individual and institutional biographies.

Moreover, tracing occurrences of a phrase can only get you so far. Robertson shows how the meanings of a phrase can differ between authors and over time. Herd immunity meant one thing to Eichhorn and Potter and something else to Topley and Wilson. Are the Americans actually part of the British lineage of herd immunity? It is not yet possible to say. Sometimes it makes more sense to trace the history of an idea and not just the phrase. That is harder to do: you have to interpret sources and make judgment calls about which antecedents are relevant. Sheldon Dudley, who studied outbreaks of diphtheria at a British boarding school in the 1920s and 1930s, was close to our modern concept (i.e., what percentage of a herd must be immune for the group to be protected). Topley entertained a broader concept, in which a clean water supply or an effective public health officer could confer herd immunity by preventing the transmission of a pathogen. In 1935 he explained that the “English herd” had achieved immunity to plague and malaria because living conditions in England had improved so that those diseases no longer circulated there.¹¹ Although Dudley and Topley used the same phrase, they did not mean the same thing.

The science of epidemiology emerged out of the effort to understand the rise and fall of epidemics. The history of herd immunity is one piece of this long discourse, but one with enormous public interest and policy relevance today. Many of us hope to resume normal lives once herd immunity is achieved—possibly even before that milestone. The United States has invested heavily in vaccines, a reflection of our long-standing search for magic bullets.¹² Countries that can afford the vaccines are already enjoying the rewards of that approach. But the history of herd immunity suggests that alternative approaches can still have value. Salmon sought herd immunity through improved nutrition. Topley advocated optimization of public health programs. Taiwan exemplified Topley’s approach: this country (and a few others) showed that COVID-19 could be controlled through strategic screening, contact tracing, and isolation. All countries should consider seriously the merits of these different ways of achieving herd immunity.

The past year has been the best of times and the worst of times for herd immunity. The new relevance of the concept has motivated substantial research that should improve our understanding of the dynamics of epidemics and our ability to model future outbreaks. But the concept has wreaked havoc on the global stage. The countries that discussed herd immunity most enthusiastically—England, Sweden, Brazil, and the United States—are overrepresented on COVID-19’s bill of mortality. Perhaps “herd immunity” is a misnomer. At a March 2021 talk in Boston, Massachusetts, George Davey Smith made the case for a different phrase. Smith recognized that herd immunity was fitting when and where it originated: with US livestock veterinarians and their herds of cattle. But Smith gave more credit to the British researchers and their experiments with mice. Greenwood, Topley, and Dudley all wrote about “herds of mice,” but the proper phrase for such a group is a “mischief of mice.” We should join Smith in wondering whether debates would have played out differently over the past year had we all been discussing “mischief immunity” instead.