Infectious diseases can cause significant harms in populations. For example, measles can spread quickly, infecting as many as 90% of those who come into contact with the disease , and killing one person in every 5000 cases in high-income countries and as many as one person in every 100 cases in low-income countries . Smallpox, marked by a 30% mortality rate, easy transmission, and no known treatment, is an even deadlier disease . Influenza infects between 10 and 20% of the population during an average epidemic, and between 40 and 50% of schoolchildren and nursing home residents, to whom it is particularly dangerous [11: 411]. Other diseases—diphtheria, mumps, hepatitis A and B, typhoid, varicella, rubella, pertussis, polio, and now COVID-19—may also represent threats to public health.Footnote 1
Individuals are not only faced with the threat of deadly harm, but due to the infectiousness of these diseases, they may facilitate the harm further. Battin et al. argue that individuals are vectors, and not just victims of infection, “embedded in a web of biological relationships” [4: 77]. Being a carrier of dangerous pathogens will in most cases introduce moral obligations for individuals, within constraints of demandingness and feasibility, to make sure not to spread them, often at the cost of their own preferences. Ignoring these obligations will often amount to “assaulting” those around us [4: 86]. Obligations may be borne by potential carriers as well; some diseases will spread quickly, and individuals will become asymptomatic carriers well before having a chance to know it. Admittedly, we may sometimes come to accept exposing others to risk at the social level. For instance, there is an overwhelming consensus in virtually all countries that we should not give up on driving simply because it carries significant potential to cause people harm. This is because driving produces significant benefits for everyone . Risks of serious, and even deadly harm may thus be found acceptable, particularly when they are outweighed by significant collective benefits, although it may often be difficult to calculate between such risks and benefits. However, risks of harm from infectious diseases, as we claim in this paper, have hardly seemed acceptable since the emergence of vaccination. According to Jeffrey Ulmer and Margaret Liu, vaccines stand out as “the most efficacious and cost-effective of global medical interventions” [32: 291]. It is owing to vaccination campaigns that once common diseases, like diphtheria, have largely disappeared from public consciousness [12: 164], while some, like smallpox, have even been eradicated.Footnote 2 In the great majority of cases, vaccinations are effective and safe, and most people take only minor risks to their health when they are administered. This is not to suggest that adverse effects from vaccines are never serious—the influenza vaccine, for instance, could cause Guillain-Barré Syndrome, which may lead to paralysis . The chances, however, of suffering serious adverse effects are, in most cases and for most individuals, considerably smaller than risking the contraction of the infectious disease. Therefore, the choices of both vaccination and non-vaccination come with a risk of harm to the choice-maker, but a prudent gambler would invariably bet on the vaccination.Footnote 3
A small minority of individuals, however, would be prudent not to bet on vaccines. Public health experts identify three groups of persons who cannot be vaccinated for medical reasons, and are thus vulnerable to infectious diseases: (1) children too young to be vaccinated (e.g., children should not undergo vaccination against measles before the age of one ); (2) persons for whom the vaccination is ineffective (normally identified as vulnerable only after showing signs of infection); (3) persons who are immunosuppressed or seriously allergic to particular vaccines (see also [29: 387, 17: 548]). For members of these groups, the costs of vaccination are significantly higher.
Luckily, and most importantly for considerations of harm, these groups can be protected by indirect effects produced when others undergo vaccination. Successful vaccination reduces the transmission of infectious pathogens within the population, making it less likely that the vulnerable will be infected. The extent to which vaccination can produce indirect effects will depend on the particular infectious disease—how transmittable the pathogen is, or the nature and duration of the immunity induced by vaccination [15: 912]—but estimations can be made of vaccination rates to be reached in the population for the spread of a disease to be minimized, and for the non-vaccinated to be protected (e.g., for measles, the rate is 92–94%). This threshold theorem is often referred to as ‘herd immunity’. Assuming a “homogenously mixed” population ([1: 641, 15: 911]), reaching a herd immunity threshold entails that the vaccinated will “serve as a protective barrier against the likelihood of transmission” [22: 264], thus also preventing outbreaks of infection. Importantly, herd immunity is not a guarantee against the infection spreading altogether, but makes it much less likely that any infected individual will come into contact with those who are vulnerable [12: 162].Footnote 4
As a public good, herd immunity is both non-excludable and non-rivalrous. If a good is non-excludable, then no one can be excluded from the benefit produced by the good. All members of vulnerable groups will be afforded protection within a population that has realized herd immunity, but so will those who elude vaccination for non-medical reasons. The claim that herd immunity is non-rivalrous means that the extent to which a vulnerable (or any other) person benefits from herd immunity is not affected by the extent to which another person benefits from it ([12: 163–164, 17: 548]).
We join theorists who hold that we have a collective harm-based moral obligation to realize herd immunity (e.g., [12, 16, 28]). For individuals, this translates into a personal obligation to undergo vaccination (or to have one’s children vaccinated), when this does not incur significant medical costs to them (or their children).Footnote 5 By doing so, the individual contributes to the collective effect of minimizing the risk of harm to members of all vulnerable groups. Conversely, the more individuals fail to vaccinate when herd immunity has been realized, the more likely it is that the good will be eroded, and that individuals (particularly those from vulnerable groups) will be susceptible to and suffer significant harms.Footnote 6
Vulnerable individuals are normally identified as belonging to one of the groups we mentioned earlier. But realizing herd immunity also carries special significance in precarious circumstances for public health—e.g., when infections threaten public life (due to the seasonality of certain diseases), or when outbreaks erupt unexpectedly and are difficult to control [33: 3123]. Herd immunity will protect anyone in such circumstances who failed to protect herself via vaccination or who is under-immunized. Of course, the obligation for individuals to collectively help the vulnerable by realizing herd immunity will not obtain in all public health circumstances. For instance, public health providers may have a record of offering bad service in terms of standards of effectiveness, safety, or transparency [33: 3125]. The moral requirement for individuals to be vaccinated, we contend, can be made only in the presence of reliable medical service, including the administration of safe vaccines.
We want to emphasize two key features of our claim. The first is that any request for exemption for not contributing to herd immunity, as hinted, must be backed by well-established medical reasons, and not by religious or lifestyle beliefs.Footnote 7 The grounding of our case for vaccination in considerations of harm should easily show why personal beliefs or their significance for holders of comprehensive views should be insufficient to secure an exemption. In this paper we will assume that, standardly, individuals are not, or should not be allowed to act upon their religious or lifestyle beliefs if this entails the risk of significant harm to others (provided the action does not produce a collective benefit that others accept). Since holders of religious and lifestyle beliefs can erode the benefit of herd immunity by refusing to vaccinate, they expose others to risk of significant harm. It is thus impermissible for individuals to dismiss their collective moral obligation to realize herd immunity on the grounds of religious and other lifestyle beliefs.Footnote 8 We later discuss whether they may request exemptions if vaccination rates for infectious diseases are well below or well above herd immunity thresholds.Footnote 9
Second, here we specifically advocate a moral, and not a legal harm-based obligation to undergo vaccination. By this, we do not suggest that harm brought about by non-vaccination can never be sufficient for establishing a program of mandatory vaccination; if anything, considerations of harm are often thought to be the most obvious driver for instituting coercive policies . Instead, we mean to say that, first, considerations of causing and preventing serious harm should in themselves strongly motivate individuals in their moral behavior. Individuals should not require the threat of having their wrists slapped by the state in order for them and their children to undergo vaccination. If this is, however, insufficient to prompt individuals, legal means may be considered, as ‘Plan B’, to ensure that the vulnerable are protected, although these would possibly have to be weighed against other important considerations, such as individual liberty and social trust. Second, even if the legal means are justified, governments may find it difficult to come up with appropriate regulation, and in a timely fashion. Such is the current state of the COVID-19 pandemic. In the absence of an established legal framework for vaccination against COVID-19, many individuals might be asking themselves whether they should vaccinate in light of some moral obligation. Our considerations here offer critical guidance for their decisions.
We now turn to showing why the individual obligation to undergo vaccination, if solely driven by realization and preservation of herd immunity, may significantly vary in strength given vaccination coverages in a population. To fully comprehend the effect of herd immunity and of each individual vaccination that contributes to it, we should examine how individual contributions relate to the herd immunity threshold. Jonathan Glover offers a broad distinction between (a) an absolute threshold, where an effect is produced only when the threshold is reached, and (b) a discrimination threshold, where every single increment produces an effect, but such that is too small to be noticed on a large scale [19: 127]. Herd immunity, we find, fits neither of these two categories neatly. Instead, the benefit of each vaccination to the non-vaccinated rises as we approach the herd immunity threshold. In other words, the value of increments peaks at the threshold for that particular infectious disease. This is because individual vaccinations, as we approach the threshold, close off the remaining ‘routes of disease’ that still threaten the non-vaccinated. Well below the threshold, on the other hand, individual vaccinations are much less significant in themselves for contributing to the collective effect, although their benefit is not completely non-existent.Footnote 10
Imagine a micro-society of ten people that could come under threat of a dangerous infectious disease, for which a vaccine is well-researched, available, and very effective at reducing transmission. Imagine that individuals X and Y are vulnerable, and cannot be vaccinated, while persons A–H can, and if they do, they will all mount the same positive response to the vaccine. Finally, imagine that X and Y rarely if ever come into contact with each other, and will not infect each other if everyone else undergoes vaccination. The starting point, however, is that no one is vaccinated. If A undergoes vaccination, this benefits X and Y, but not significantly, because they can still contract the disease from seven other persons, B–H. In numerical terms, A’s vaccination reduces the probability of infection by 12.5%. If B follows A’s example, the benefit is once again small, but greater than when only A vaccinated, since there are now fewer persons who may infect X and Y as a result of B’s vaccination, i.e., this reduces the probability of infection by 14.3%. Fast forward to the final person vaccinating—H. If H vaccinates, assuming all other non-vulnerable persons have vaccinated before him, then all the sources of disease for X and Y will have been cut off, and the reduction of the probability of infection will have reached 100%. Thus, H’s vaccination is most significant to X’s and Y’s benefit; although persons A–G have diminished the risk of infection in the micro-society, it is H’s vaccination that puts the stamp on deterring the threat to X and Y, and, we believe, produces the strongest moral obligation.
Strictly from the perspective of herd immunity, we believe that the strength of an obligation to vaccinate for the sake of the vulnerable should track the benefit that the single vaccination produces for them. Since the individual benefit of H’s vaccination to X and Y is significantly greater than that of A, H seems to have a stronger obligation to vaccinate than A. Marcel Verweij hints at this kind of reasoning, stating that “if most people forgo vaccination against influenza, the effects on public health of my choice for vaccination will become negligible […] (I)f non-compliance is common, my obligation to contribute to prevention will weaken or even fade away” [34: 329–330]. In a similar way, Derek Parfit has once claimed that individuals should contribute to some collective benefit only if they believed that enough of their peers will act in the same way [26: 77]. If there is no coordination among those potentially undergoing vaccination, and there are few contributors, then from the perspective of realizing herd immunity individuals have a weak moral obligation to vaccinate.Footnote 11
Above the herd immunity threshold, some believe that the individual obligation to vaccinate evaporates completely. Angus Dawson believes that once the herd immunity threshold has been reached, further individual vaccinations produce no additional benefit (and may, in fact, incur a cost), and should therefore not be obligatory [12: 171–177]. Roland Pierik claims that, since herd immunity is sufficient to protect public health, some exemptions to vaccination could be justifiable, if a practicable and justifiable exemption system could be devised [28: 226–228]. Thus, even if we maintain that individuals are obligated to vaccinate just above the threshold, in order to keep the population ‘above water’, current harm considerations expressed through the value of herd immunity do not explain why individuals should vaccinate well above the herd immunity threshold.
We turn to explicating two compelling arguments grounded in harm that complement the obligation to realize herd immunity. We start from the obligation to vaccinate below the threshold.