Immunities of the Herd in Peace, War, and COVID-19.

Intermittently, the concept of herd immunity has been a potent, if sometimes ambiguous and controversial, means of framing the global response to the COVID-19 pandemic and envisaging its end. Realizing the full meaning of human herd immunity requires further attention to its connections after World War I with British social theory. Distracted by "obvious" yet unsubstantiated correspondences with veterinary research, historians of the concept have not engaged with the more proximate influence of discussions of social psychology and group dynamics on postwar epidemiology. Understanding the openness of early 20th century epidemiology to social thought deepens our appreciation of the significance of herd or population immunity, as well as suggests new avenues for exchange between public health and contemporary social sciences. (Am J Public Health. 2022;112(10):1465–1470. https://doi.org/10.2105/AJPH.2022.306931)

One hundred years ago, there was no formal concept of human "herd immunity." When influenza swept across the world in 1918–1919, the rise and fall of the pandemic were not explained in relation to the absence or presence of herd or population immunity. The notion of specific personal immunity had caught on only 30 or so years earlier, displacing older and looser impressions of an individual's constitutional resistance or susceptibility to disease.[1] Accordingly, immunity immediately after World War I still referred to a person's reaction to infection, the singular response of the body's defense mechanisms to contact with the foreign and unfamiliar.

Until British epidemiologists W. W. C. Topley and G. S. Wilson coined the term "herd immunity" in 1923, public health officers struggled to describe and to frame how human collectives might eventually become invulnerable to epidemic disease.[2] It was not until the 1920s, particularly through Sheldon F. Dudley's epidemiological studies, that herd immunity gained currency as a conceptual tool tracing the human population's shifting immunological terrain. But herd immunity then signaled social gregariousness and altruism, generative mutualism, and a spectrum of collective protections—not a simple threshold of past infection and vaccination as it so often does now.

At the beginning of 2020, with the global spread of the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), herd immunity became a common, if often ambiguous, simplistic, and controversial, locution. Before much was known about the virus's infectivity and virulence, some public health leaders and politicians hoped that mitigated transmission might quickly result in natural herd immunity, when enough of the population had been infected and recovered. Once it was clear that such a policy would lead to excessive deaths and collapse of health care systems, herd immunity often became a term of reproach, signifying state indifference to the survival of its citizens. Instead, most governments articulated a commitment to suppression or even elimination of the virus until artificial herd immunity, through immunization, could be achieved. Unaware of actual historical usage of the term, many experts felt uncomfortable with the simple veterinary analogies they presumed it implied. Epidemiologists and politicians frequently disavowed any hankering for natural herd immunity; various activists demanded to be "unherded." There was a misapprehension that a herd could only refer to collectives of nonhuman animals, and that it must, therefore, be demeaning.

My intention here is to reveal more clearly a little of the changing conceptual landscape of collective immunity, thereby contributing to making our understanding of epidemic sociality and solidarity more robust and usable. What did it mean in the 1920s to imagine the immunity of a human herd? What lessons for contemporary public health can be gleaned from these earlier conceptual frameworks? Recent historical accounts have gestured toward veterinary similes of herd immunity, emphasizing the power of animal symbolism[3] or multispecies crossovers and ecological mindsets.[4] Of course, it would be silly to deny that alluding to a herd signifies some connection with animal husbandry.

But I want to explore the more proximate link to popularity of the herd—not just as a convenient veterinary analogy—in nascent social psychology, particularly in the study by surgeon and social theorist Wilfred Trotter, Instincts of the Herd in Peace and War (1916).[5] Trotter's investigation of human gregariousness, altruism, corporate morale, and suggestibility exerted profound influence in the 1920s on psychoanalysis, sociology, and British epidemiology—even if the work is largely forgotten now. It made the herd a supple and powerful metaphor for the social dynamics and connectedness of human populations, a functional means to describe altruism and self-sacrifice. In view of the many uses and abuses of herd immunity in recent years, there is clearly much at stake in getting this history right.

The debate over herd immunity against COVID-19 was especially fierce during 2020 in Britain, where the concept had originated. Sir Jeremy Farrar, director of the Wellcome Trust (now simply Wellcome), remembered that at the start of the year "herd immunity stampeded on to the scene. It caused a public outcry." An infectious diseases physician, Farrar served in the government's Scientific Advisory Group for Emergencies (Sage). In March 2020, he was incredulous on hearing Sir Patrick Vallance, the chief scientific advisor, casually mention that Britain was pursuing a strategy to attain herd immunity through natural infection. Farrar believed that seriously considering "such an idea three months into a new disease beggared belief."[6] As British journalists Jonathan Calvert and George Arbuthnot later put it, "herd immunity was a view that appears to have infected the government."[7]

And yet, before long, when the scale of the threat was obvious, everyone denied having ever contemplated nonintervention, letting the virus rip through the community. Rather, Sage came to suggest a series of intermittent lockdowns and other social and behavioral changes to reduce transmission and lower mortality rates in advance of effective vaccination. "It was still, technically," Farrar admitted, "a managed herd immunity strategy but over a longer period of time that would leave fewer deaths in its wake."[8] Containment or elimination was out of the question, leaving eventual population immunity through natural and artificial means as the clearest route to ending the pandemic. But, for most British scientific advisors and politicians, "herd" had become a dirty word, rarely to be uttered in public.

All the same, blatant aspirations for herd immunity were not entirely abandoned during the first year of the pandemic. The desire to attain natural herd immunity, before widespread vaccination was possible, became a particular enthusiasm of libertarian, free-market groups and some fringe or ostentatiously heterodox biomedical researchers. Thus, in October 2020, an assembly of scientists drafted a statement proposing "focused protection" of those most at risk from COVID-19 while others resumed normal lives and exposed themselves to infection to build herd immunity.[9] Originated at the American Institute for Economic Research, a right-wing thinktank in Great Barrington, Massachusetts, the declaration considered neither the resultant stresses on health care systems, nor the difficulties of long-term segregation of the aged and infirm, nor the possibly limited duration of any postinfection immunity. It signaled to some readers that the idea of herd immunity had been captured by reactionaries indifferent to human suffering, perhaps bent on culling the "unfit" from the population.

Several conventional public health experts perceived the taint of eugenics, or doctrines promoting survival of the fittest, in these herd immunity strategies. Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, warned that "never in the history of public health has herd immunity been used as a strategy for responding to an outbreak, let alone a pandemic." He was convinced that true population immunity could be reached safely only through future vaccination: thus, "herd immunity is achieved by protecting people from a virus, not by exposing them to it."[10] In a further riposte to the Great Barrington Declaration, William A. Haseltine, virologist and biotech entrepreneur, asserted that "herd immunity is another word for mass murder."[11] Other researchers denounced the natural herd immunity game plan in the John Snow Memorandum, published in The Lancet, stating it was "a dangerous fallacy unsupported by the scientific evidence."[12] Herd immunity had come to imply a sort of microbiological dystopia, nature red in tooth and claw.

During 2021, the advent of a more readily transmitted but supposedly less virulent strain of SARS-CoV-2, along with augmented vaccine coverage in wealthy countries, raised hopes that higher rates of population immunity would allow greater control of the pandemic. This optimism was predicated on a process of herd immunity, yet few experts were prepared any longer to acknowledge it publicly. Journalists sometimes speculated on what levels of Omicron variant spread and vaccine uptake were required to end COVID-19 in their nations, but as the pandemic persisted, most avoided explicit debate about the virtues and harms of herd immunity.[13] Instead, "opening up," "lifting restrictions," "living with COVID," and "vaccine rollout" became acceptable euphemisms. Yet such herd immunities, ever less spoken, of the COVID-19 pandemic were quite distinct from the concept of herd immunity that had circulated some 100 years earlier.

Writing in 1919, toward the end of the influenza pandemic, William W. C. Topley lamented that "circumstances of the past year have rendered research work of any kind far from easy." At Charing Cross Hospital, London, the local response to the scourge—the agent of which was still undetected—had impressed upon the young epidemiologist that most microbiologists were "more concerned with the minute investigation of comparatively small samples of cases than with the broad view of an epidemic of disease as a biological process."[14] How, he asked, might one explain the characteristic wave form of the epidemic—its rise, cresting, and subsidence? Intrigued, like so many of his colleagues, by variety and diversity within bacterial species,[15] Topley wondered if increased pathogenicity or virulence of the microorganism or parasite could disturb the equilibrium with its host, thus inciting an epidemic. Moreover, he suspected that "the outstanding feature in the subsidence of an epidemic is the loss of infectivity by the bacterial virus."[16] At the time, he preferred this interpretation to speculations about changes in host resistance and alterations in the surrounding environment. Nonetheless, the correct answer to such a puzzle in population immunity would await the results of laboratory experiments in epidemiology, which he was proposing to conduct.[17]

Observation of his mouse colonies soon caused Topley to reassess what gives rise to epidemics. In 1923, he reported from his new post at the University of Manchester that when he added "susceptible individuals" to a mouse population previously in equilibrium with the bacterial parasite, a wave of new cases quickly broke out. As the susceptible mice died or recovered, the epidemic declined.[18] His attention turned to the immune pattern of the host population. The same year, with junior colleague Graham S. Wilson, he began to ponder "the question of immunity as an attribute of the herd." They noted that as the proportion of immune individuals—resistant after vaccination or surviving infection—rises in a population, a limited number of susceptible individuals also might be protected from disease.[19]

After moving to the London School of Hygiene and Tropical Medicine in 1927, Topley joined his old friend Major Greenwood, Britain's premier epidemiologist, in a series of experiments to determine the distribution of immunity across mouse populations. They tended to discount the logistics of germ transmission, focusing instead on the rapidly evolving character and quality of the herd, examining how resistance and susceptibility were figured dynamically across the host population. Distinguishing their approach from common interwar obsessions with breeding and eugenics, they highlighted a range of social and "educational" factors influencing herd immunity, rejecting hereditary or genetic contributions.[20] "Nothing has emerged from our researches," Topley et al. wrote in 1930, "to suggest that under any conditions of selection or immunization, environmental factors, in the sense of quality and quantity of infection, would become negligible."[21] Topley was principally interested in how broad histories of exposure and contact, sometimes along with vaccination, might reshape or re-educate the immunological profile of his experimental herds. Thus, the herd had become a socio‒biological formulation, shorthand for community, or possible co-immunity—not signifying bare life to be culled by infection.

At the Royal Naval Hospital, Chatham, England, pathologist Sheldon F. Dudley began to investigate communal aspects of immunity among schoolboys in the 1920s, thereby transferring the notion to practical studies of human populations. Friendly with Topley and Greenwood, Dudley was eager to explore herd immunity in human groups, tracing the parallels of mice and men. In 1924, he observed that when diphtheria broke out among boys in a dormitory of the Royal Hospital School in Greenwich, England, known as the cradle of the British navy, the extent of population immunity from past infection and vaccination set the level of spread of the bacillus to both the protected and the unprotected. Thus, he wrote, "the degree of herd immunity determines ultimately the disease-producing power of the biological agents of illness."[22]

Having trained in tropical and naval medicine, Dudley was acutely aware of the importance of ecological and sociological reasoning in accounting for patterns of infectious disease. The notion of the herd seemed an especially effective way to imprint an ecological attitude on epidemiology. He believed it was

only by approaching human infection from the ecological point of view, and looking at epidemics as manifestations of a loss of balance between the mutual adjustment of host and parasite, that the natural laws controlling periodicity, extent and malignancy of diseases of herd and individual will finally be defined.

British psychologists and social theorists had taught him—and Topley and Greenwood, too, he was sure—that the herd was a suitable model for gregariousness and cooperative action within human populations. "The prevention of disease in herds and individuals," Dudley concluded, "necessitates a proper understanding of their evolutionary biology and psychology."[23]

Topley had learned from Wilfred Trotter's Instincts of the Herd in Peace and War about the emotional force exerted in collective interaction, the need to respect humans as social animals, as gregarious as other herding animals. He read his surgical colleague's "admirable essay" soon after its publication and kept it close the rest of his life.[24]Instincts of the Herd, wrote Topley, "illuminated and reoriented our mind in such a way that it has thereafter a value peculiar to itself." He felt "it will remain our bed-rock"[25]—a book that "seems fundamental to us."[26]

Dudley, too, recalled that it was Trotter who had shown his generation of epidemiologists that the herd is "an apt symbol to use in describing the attributes of human groups, because it emphasizes the biological truth that there is so much that is strangely similar and familiar in the behaviour of all gregarious animals."[27] While social anthropologist Bronislaw Malinowski may take those referring to herds to task for assuming human society correlates with a biological collective,[28] Dudley and other medical scientists in London dismissed such reservations. It was Trotter's invocation of the herd instinct during the war that made them realize "the truly overriding and irresistible influence of the community and group on the character of the individuals who compose the community and group." For Dudley, the herd instinct, as he put it, "fulfils a most essential function in preserving the stability and status quo of society and inhibiting any impulse which members of the herd may acquire or possess to run 'after false Gods,' cranks, and new ideas."[29] The herd instinct, he suggested, gives us altruistic social measures and mass vaccination, from which derives herd immunity. Thus, the operation of the herd produces assent and submission, or common resistance, rather than inscribing an index or threshold of protection to be achieved.

An accomplished head-and-neck surgeon who dabbled in social theory, Trotter was a star in the elite London medical firmament. An aloof figure at University College Hospital, London, he rarely left the vicinity of Gower Street, though for a time he became the King's surgeon (and Sigmund Freud's last surgeon). Married to the sister of psychoanalyst Ernest Jones, Trotter engaged critically with Freud's early work, stimulating the Viennese psychiatrist to apply his insight to collective behavior. In 1908, on his return from the first international psychoanalytic congress in Salzburg, Austria, Trotter had begun writing on herd instinct for the new Sociological Review, arguing for "gregariousness as a phenomenon of profound biological significance."[30] Unlike sociologist Gustave Le Bon, who had recently described the morbid nervous excitement of the crowd,[31] Trotter chose to extol the herd's homogeneity, camaraderie, suggestibility, and fear of alienation, which he claimed brought about modern civilization, national ideals, and self-sacrifice. According to Trotter, "the only medium in which man's mind can function satisfactorily is the herd, which therefore is not only the source of his opinions, his credulities, his disbeliefs, and his weaknesses, but also of his altruism, his charity, his enthusiasms, and his power."[32] Our instinctive herding, he believed, is the conduit for all intercommunication and connectivity, the condition of associated life and flourishing.[33]

The chauvinism of nations in World War I sharpened Trotter's perceptions of the human herd instinct. In new essays published in the popular Instincts of the Herd in Peace and War in 1916, he expatiated on biological mass psychology, or "anthropocentric biology" as he called it. The herd instinct—"the stamp of being regulated by the existence and influence of his fellows"—means the target of natural selection shifts from the individual to the collective, with the herd constituting a sheltering organism, conferring a kind of immunity. Ease of intercommunication and the prevailing sense of altruism, Trotter believed, enable "the herd to act as a single creature whose power is greatly in excess of the sum of the powers of its individual members."[34] It was this concept of the human herd as a superorganism with distinctive and widely encompassing immunological or protective capabilities that most impressed postwar epidemiologists.

Others seized on Trotter's argument for the homogeneity and suggestibility of human herds and their need for aggressive leadership. The herd instinct is responsible for social morale, which "gives smoothness of working, energy and enterprise to the whole national machine, while from the individual it ensures the maximal outflow of effort with a minimal interference from such egoistic passions as anxiety, impatience, and discomfort."[35] No wonder that Edward Bernays, Freud's nephew and founder of public relations; Joseph Goebbels; and Benito Mussolini expressed admiration for Trotter's theories.[36] His influence thus extended well beyond epidemiology. Trotter also inspired one of his medical students, Wilfred Bion, to take up psychoanalysis and develop programs in "group dynamics." Reading Trotter's speculations on the herd in peace and war later stirred Elias Canetti to write his magnum opus Crowds and Power (1960).[37]

The concept of herd immunity emerged after World War I congruent with a new theory of human bio-sociality. It was the product of passage from animal biology to human social thought, then across to the equally permeable working knowledges of immunology and epidemiology. This is not the place to trace the fortunes of herd immunity over the past hundred years, leading up to its various consummations in the current pandemic. Evidently, the bonds of herd immunity with an old, forgotten social theory have become attenuated. But perhaps not completely sundered.

The most telling example is surely the celebrated 1985 article by Roy M. Anderson and Robert M. May modeling attainment of herd immunity. Although it concentrated on figuring out the necessary level of vaccination coverage, building on recent successes of smallpox eradication and childhood immunization programs, the bio‒social configuration of the herd was deemed equally valuable. Reaching a threshold vaccinated proportion of the group is not all that matters. Herd immunity depends also on the "degree and intimacy of contacts among people and the prevailing levels of genetic, spatial and behavioural heterogeneity in susceptibility/resistance to infection."[38] In Anderson and May's framing, the herd's social and motivational qualities—its gregariousness and communicability—remain crucial in getting to population immunity. Thus, Trotter's ghost continued to haunt epidemiology, albeit intermittently.

It seems we have a choice. Rather than be content with resorting casually to worn and facile veterinary metaphors and folk social theories, or searching for simple levels and thresholds, we might properly return sociological complexity and ethical nuance to our apprehension of herd immunity, drawing more deeply, as did Topley and Dudley and their colleagues, from Trotter's social psychology and moral imagination. If we must carry the burden of epidemiologies past, let us be sure it is decent conceptual baggage, not just a basket of empty metaphors. Or, more radically, we could simply consign the herd, whether instinctual or immunological, to history. In the past hundred years, the social sciences have moved on, offering even more compelling perceptions and insights that can be harnessed to explain what population immunity—indeed, what "population"—means today.[39] Maybe it is time, sociologically, to unherd.

I am grateful to Anne Kricker and Hans Pols for comments on an earlier version of this article. Conversations with David Jones and David Robertson also shaped my thoughts on the topic. Archivists at Wellcome kindly allowed me access to Topley's uncatalogued collection.

The author reports no conflicts of interest.