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Several theoretical studies have defined the immunization decision as a trade-off between the perceived utility of immunization and non-immunization (Bauch & Earn, 2004; Bauch et al., 2010; Betsch et al., 2013; Brewer & Fazekas 2007; Galvani et al., 2007; Manfredi et al., 2009; Nguyen et al., 2011). The utilities are thereby defined as the difference between expected costs of contracting the virus and side-effects of vaccination, while the costs are either the product of the severity and the probability of infection or the product of the severity and the probability of side-effects. A rational individual would only get immunized if the perceived utility of immunization is higher than the expected utility of infection. Immunization, however, does not only yield a direct effect on reducing the risk of infection, but it also has an indirect social effect of reducing the transmission of the virus, called herd immunity (Fine et al., 2011).
Assuming that the cost of immunization is independent of the number of individuals getting vaccinated, making this indirect effect of immunization salient could influence the perceived utility of immunization and non-immunization. There have been several studies looking at the effect of communicating the social benefit of immunization on attitudes and intentions (see Hakim et al., 2019 for a recent systematic review). Individuals whose utility does not only depend on their wellbeing but also on those of their social environment perceive the utility of immunization as higher when herd immunity is known (Shim et al., 2012; Betsch et al., 2013). Differently, communicating herd immunity can also decrease immunization intentions for self-serving individuals (Dawes, 1980). The awareness of herd immunity increases the perceived utility of non-immunization for these individuals as they believe that more people would get immunized (Hershey et al.1994; Bauch & Earn, 2004; Manfredi et al., 2009; Betsch et al., 2013). As the risk of infection reduces with the number of individuals immunized, the individual benefit of getting immunization decreases. Individuals may, therefore, decide to free-ride and profit from herd immunity, avoiding individual costs of immunization (Fine et al., 2011; Betsch et al., 2013). Empirical evidence appears consistent with these conjectures. Research using experimental surveys has shown that communicating herd immunity can increase free-riding behaviour when the message emphasized the individual benefit of others getting immunized (Betsch et al., 2013). Differently, several studies have shown that explaining the concept of herd immunity can increase the willingness to be vaccinated in western countries and when the cost of immunization is low and non-vaccinators have low responsibility (Arnesen et al., 2018; Betsch et al., 2017, Böhm et al., 2019). While these results suggest that herd immunity influences immunization intentions, it is not clear how the definition of its beneficiaries influence the reaction to the message. While experimental studies on social preferences have shown that individuals are motivated by the wellbeing of others, individual may hold social identities at various levels of abstraction, ranging from concrete groups of individuals (e.g., own friends and family) to broader categories of individuals such as citizens of their country (Andreoni et al., 2008; Charness & Gneezy, 2008). Previous studies on social preferences have shown that individuals exhibit more altruistic preferences if they know more about the potential recipients, such as their social belonging (Eckel & Grossman, 1996; Bohnet & Frey, 1999; Burnham, 2003; Charness & Gneezy, 2008). Similarly, other studies show that altruistic behaviour is negatively related to social distance; e.g. whether beneficiaries are close friends or not (Jones & Rachlin, 2006; Leider et al. 2009; Goeree et al. 2009). As such, individual intention to get immunized for the benefit of others may depend on the social distance between the decision-maker and the beneficiaries of herd immunity.