Lakhasly

Herd immunity is often invoked as an ethical justi- fication for vaccination and/or coercive vaccine mandates.1–3 However, the term ‘herd immunity’ has multiple referents,4 which can result in ambi- guity, including regarding the role of herd immu- nity in ethical arguments and policy debates. For example, ‘herd immunity’ can refer to (1) the herd immunity threshold (HIT) at which models predict the decline of an epidemic, sometimes even- tually resulting in elimination; (2) the percentage of a population with immunity, whether or not it exceeds a given threshold; or (3) the indirect benefit afforded by (the sum of) individual immunity to non-immune people (also known as a ‘herd effect’).5
The accumulation of immune individuals in a population (herd immunity in the second sense above) can lead to two different outcomes. First, for pathogens such as measles or smallpox where immunity from infection or vaccination is highly effective at preventing (re)infection (sometimes referred to as ‘sterilising’ immunity), accumulation of immune individuals can result in elimination, that is, the sustained reduction of local transmis- sion to zero. Maintaining R, the average number of secondary cases per infection, below 1 by keeping
the immune fraction above a particular ‘herd immu- nity threshold’ (herd immunity in the first sense), is often understood to be a necessary condition for elimination in such circumstances.6
Second, for pathogens where immunity from infection or vaccination is relatively ineffective at preventing subsequent (re)infection, accumulation of immune individuals results in the development of an endemic equilibrium.7 Rather than being elim- inated, such pathogens continue to circulate, often mutate, and (re)infect members of the population whose immunity wanes over time; examples include respiratory syncytial virus (RSV), influenza viruses, seasonal coronaviruses and SARS-CoV-2 .7–9 The absence of elimination should not be confused with the absence of herd immunity, and indeed the term ‘herd immunity’ is often used to describe the accumulation of immunity in a population due to endemic viruses.8
In the endemic equilibrium case, herd immu- nity (in the second sense) nevertheless produces important benefits. For example, average severity of disease will be lower due to the sum benefits of individual immunity. This is what makes the differ- ence between the high total mortality in the first wave of a pandemic (eg, COVID-19) or epidemic (eg, smallpox in the newly colonised Americas)10 and the lower mortality in subsequent waves, even in the absence of vaccination.11 There may also be some indirect protection of non-immune individ- uals (herd immunity in the third sense above) but this is often a relatively minor phenomenon for many pathogens because reinfections of immune individuals, although asymptomatic or minimally symptomatic, are often transmissible.12 13
Ambiguities about the referent(s) of the term ‘herd immunity’ in a particular context have the potential to undermine the validity of ethical arguments, including those which inform policy decisions. Specifically, where it is claimed that indi- viduals have a moral obligation to contribute to herd immunity this may be used to justify coercive policies (ie, if one is obligated to do something one is less entitled to not do it, and so coercion is more acceptable from an ethical standpoint). Since the meaning of a term is arguably partly determined by its use, and since usage may vary, our intention here is not to argue that one particular meaning of the term is correct and the others mistaken.4 However, we will analyse two case studies to show that (1) the extent to which one has a moral obli- gation to contribute to herd immunity depends on the meaning of the term herd immunity, (2) the claim that the benefits of reaching an HIT entail special moral obligations is often misguided, and (3) that moral obligations to contribute to herd immunity in an endemic equilibrium are weak or non-existent.