Before the emergence of the global COVID-19 pandemic, immunity had become a relatively abstract concept in the industrialized world. For those who had easy access to health care and functioning immune systems, formerly life-threatening diseases like measles, polio, diphtheria, and influenza had been reduced to acronyms associated with vaccine schedules: MMR, IPV, DTaP, and Hib, among others. And yet even as the vaccine era dominated public health responses to infectious diseases, pockets of vaccine resistance persisted, and new diseases began to loom large in the cultural imagination: Zika, West Nile, SARS, Ebola, H1N1, and HIV/AIDS seemed to elude treatment and cure, raising the specter of global apocalypse. By mid-2020, COVID-19 embodied the worst fears and greatest promise of modern public health: a contagious disease that spread rapidly through casual social interactions but for which an effective vaccine could be developed, tested, and deployed in record time. By early 2021, the promise that vaccination would mean a quick return to prepandemic life was shattered: even as tens of millions of people were vaccinated in the United States alone, suspicion of governments and private corporations hardened into persistent anti-vaccine conspiracy theories. As vaccination rates plateaued, so did resistance to the SARS-CoV-2 virus. By mid-2021, new variants continued to evolve as waves of infection put pressure on US health systems, wreaking havoc on economic infrastructures and leading to distressing levels of mortality. In what follows, we will consider the complex relation of biological, political, historical, and personal factors that make a seemingly simple idea like immunity such a critical and controversial concept in modern society.

How, one might ask, does a society that possesses a clear solution to a public health crisis fail to follow through on it? How does the very idea of immunity expose political and cultural fault lines in a community? To explore these questions, I will set COVID-19 aside for a moment and examine the history and meaning of immunity itself. Doing so will not add to the development of practical treatments and medicine, nor will it add to debates about the effectiveness of vaccines. And yet reflections by literary and cultural critics as well as historians working in the medical and health humanities help clarify the core tensions between individual and communal rights that lie at the heart of modern public health debates. Rather than resolving questions about the limits of state power, they underscore how rhetorical and ideological structures determine the historical equity and effectiveness of public health policy. This history begins with an acknowledgment that the word immunity (immunitas) existed two thousand years prior to its use in medicine. An idea firmly rooted in Roman law, it was used to describe the exemption from tribute and taxation that a sovereign power could grant to an individual or state. In other words, an individual who was granted immunity to a certain tax did not have to pay it. Immunity remains a key concept of civil and criminal law, but it has long since expanded to signify a protection from liability or prosecution. Although immunity would not be used in medicine until the modern era, physicians have known that individuals could be susceptible or resistant to certain illnesses for millennia. However, it was only in the age of microbiology—the period that begins roughly with the work of John Snow and Filipo Pacini on cholera and of Louis Pasteur on anthrax and rabies in the late nineteenth century—that medical practitioners began to feel the need for a word that could internalize the idea of resistance to infectious disease (i.e., that immunity comes from within one’s own body rather than from an external force like God).

There is no inherent reason that medicine should have settled on immunity as its word for resistance to disease, but immunity’s early association with law and exemption proved to be important for several reasons. First, because regardless of how accurate or sophisticated the modern understanding of human immune systems is in helping develop new vaccines and medical therapies for rare diseases, the term’s prehistory reminds us that medicine’s appropriation of it emerged from an analogy to the law. Acknowledging this analogical character should prompt us to resist the essentialism that otherwise lurks at the heart of modern Western medicine—an essentialism made concrete in immune-system models of the human body that define health as a private function of individual persons (my body and my health are my own).

Second, immunity’s legal origins tell a story about the role of sovereignty in Western medicine. While the biological concept of immunity has no obvious association with forms of sovereignty that define a state’s political and legal powers, the medical profession’s appropriation of immunity as a controlling metaphor for resistance and susceptibility to infectious disease may in fact be rooted in history. Taking the long view, Western history and literature are filled with examples of writers who portray epidemics and pandemics as punishments meted out by sovereign deities against sinful populations. Consider Apollo’s plague at the beginning of the Iliad or the plague of Thebes at the start of Oedipus. Or the European justification narratives that represent devastating New World epidemics as acts of divine providence to support their colonial dispossession and appropriation of Indigenous lands. Or even the modern jeremiads that figure contemporary epidemics like HIV/AIDS as divine punishments for national sins.

Modern medicine can of course make sense of such events by pointing to interactions between pathogens, vectors, modes of transmission, and differential susceptibilities, but it would be naive to imagine that the accuracy of those explanations somehow erases the ideological foundations of medical narratives that themselves date back to at least the ancient Greeks. So that if divine sovereignty no longer has purchase over Western medicine, immunity models of susceptibility and resistance may still internalize political sovereignty in the forms of its narratives. Looking back to justification narratives, for example, once divine sovereignty lost its appeal as a model for explaining Indigenous susceptibility to contagious diseases, narratives centered on the apparent immunological and genetic inferiority of Indigenous People (the so-called virgin soils model) became dominant (see Crosby 1976; D. Jones 2004). No less appealing in their simplicity than those earlier narratives, virgin soils models substituted biological determinism (the death of Indigenous People from exposure to European pathogens was inevitable) for divine sovereignty and fed into historical colonialist tropes by flattening the relation between epidemics and complex demographic patterns.

This seemingly narrow debate about the structure of historical epidemiological narratives is critical to struggles over Indigenous sovereignty and reclamations precisely because of the state’s ongoing interest in regulating health. More pointedly, where governments determine how to define immunity, public health policy becomes the embodiment of sovereignty and political power. This dynamic becomes clearer to broader public audiences in the wake of COVID-19. During the vaccination phase of the pandemic, vaccine mandates became avatars of state power. They stood as gateways to employment, to schooling, to travel, and to the enjoyment of public spaces. Even as people resist vaccination for a number of reasons, the state’s sovereign interest in mandating vaccines arises because they help moderate the risk of epidemical outbreak via a concept known as herd immunity. In contrast to personal immunity, herd immunity is expressed as the percentage of a given population that is resistant to infection at a specific point in time. For example, when a community has a high level of herd immunity to a disease (say, 90 percent), it has a low risk of epidemical outbreak because contagious individuals have a low likelihood (one in ten) of randomly encountering and passing the pathogen on to susceptible people. On the other hand, communities with low levels of herd immunity (say, 10 percent) are at a high risk of epidemic outbreak for exactly the opposite reason. Though they share a term, an important difference between individual immunity and herd immunity is that immunity tends to be a relatively static state for individuals, whereas herd immunity changes day by day as people are born, die, migrate, or develop immunities of their own through vaccination and exposure. Herd immunity is an important tool in public health because large swings in the ratio of susceptible to unsusceptible people have dramatic effects on the likelihood that epidemics will erupt or peter out in a population.

An important point to clarify about public health policies during early phases of the COVID-19 pandemic is this: among the various strategies for combatting the outbreak, herd immunity was used as loose shorthand for policies that sought to increase rates of resistance to the SARS-CoV-2 virus by allowing it to circulate freely. As people became infected and recovered, they presumably developed their own resistances and contributed to higher levels of herd immunity against the virus. Notwithstanding the fact that as of late 2021, the estimated rate at which herd immunity becomes an effective barrier against COVID-19 (the herd immunity threshold) is believed to be well above 80 percent, the social cost of such policies is that mortality is considerably higher in the short term than it would be with strict mitigation efforts like quarantines, lockdowns, face masks, and social distancing (see Aschwanden 2020). Furthermore, as the virus circulates freely in the world, the risk of new, more infectious, and vaccine-resistant variants evolving also rises. This particular shorthand glosses over the fact that herd immunity is a key component of mitigation strategies that aim to keep infection rates low until effective vaccines can be developed and distributed to the public—at which point they would help raise overall rates of resistance. Though both of these strategies rely on herd immunity, the former proved to be a failure based on total and per capita mortality; its use among critics of quarantines and lockdowns was a euphemism for state-sanctioned policies whose direct effect was more sickness and death.

Pointing out the multiple entanglements of medicine, culture, politics, and law in the term immunity is not a criticism of the tremendous advancements that medicine and science have made over the past century. Instead, situating immunity within its larger history as a word and an idea shows how our understanding of resistance and susceptibility to infectious disease reflects larger political and cultural trajectories in the field of medicine and public health. Working with this longer temporal horizon reminds us that before the era of professionalization and specialization in the late nineteenth century, fields like law, theology, medicine, history, philosophy, art, and literature were not nearly so distinct, and words like immunity drew promiscuously from them. While immunity may conjure the disciplinary expertise of health professionals (like epidemiologists, immunologists, and infectious disease experts) in the twenty-first century, the role of the medical and health humanities in revisiting the histories of such words is to highlight the interdisciplinary entanglements that have always been part of public health and to offer insights into the narratives and forms that modern public health debates share with their predecessors.

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