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Most vaccines protect both the vaccinated individual and the society by reducing the transmission of infectious diseases. In order to eliminate infectious diseases, individuals need to consider social welfare beyond mere self-interest-regardless of ethnic, religious, or national group borders. It has therefore been proposed that vaccination poses a social contract in which individuals are morally obliged to get vaccinated. However, little is known about whether individuals indeed act upon this social contract. If so, vaccinated individuals should reciprocate by being more generous to a vaccinated other. On the contrary, if the other doesn't vaccinate and violates the social contract, generosity should decline. Three preregistered experiments investigated how a person's own vaccination behavior, others' vaccination behavior, and others' group membership influenced a person's generosity toward respective others. The experiments consistently showed that especially compliant (i.e., vaccinated) individuals showed less generosity toward nonvaccinated individuals. This effect was independent of the others' group membership, suggesting an unconditional moral principle. An internal metaanalysis ( n = 1,032) confirmed the overall social contract effect. In a fourth experiment ( n = 1,212), this pattern was especially pronounced among vaccinated individuals who perceived vaccination as a moral obligation. It is concluded that vaccination is a social contract in which cooperation is the morally right choice. Individuals act upon the social contract, and more so the stronger they perceive it as a moral obligation. Emphasizing the social contract could be a promising intervention to increase vaccine uptake, prevent free riding, and, eventually, support the elimination of infectious diseases., Competing Interests: The authors declare no competing interest., (Copyright © 2020 the Author(s). Published by PNAS.)

Background: More and more countries are discussing the introduction of mandatory vaccination policies. Yet, little is known about individuals' actual preferences for voluntary vs. mandatory vaccination policies, and the psychological processes underlying such preferences. Objective of the present research was to investigate the development of individual preferences for voluntary and mandatory vaccination policies., Methods: We conducted a controlled laboratory study (N = 168) using a repeated interactive vaccination game with decision-contingent monetary incentives. In each round, participants decided in favour of either a voluntary or a mandatory vaccination policy, followed by the vaccination decision (voluntary policy) or vaccination (mandatory policy) which both resulted in actual (financial) consequences., Results: We observe large heterogeneity in participants' preferences for the voluntary vs. mandatory policy. Under voluntary vaccination, the preference for the mandatory policy increased with decreasing vaccination rates (and increasing risk of infection). In contrast, experiencing vaccine-adverse events under mandatory vaccination increased the preference for the voluntary policy. The latter effect was larger for individuals with a negative (vs. positive) attitude toward vaccination. Overall, as individuals gathered experiences under both the voluntary and the mandatory policy, the preference for voluntary vaccination policy increased over time., Conclusions: Individuals are more willing to accept mandatory vaccination policies when vaccination rates are low. In the long run, the occurrence of vaccine-adverse events may spark opposition to mandatory vaccination., (© The Author(s) 2019. Published by Oxford University Press on behalf of the European Public Health Association. All rights reserved.)

Vaccination provides direct protection for the vaccinating individual and indirect protection for other, unvaccinated individuals via herd immunity. Still, some people do not get vaccinated-either because they cannot (e.g., due to health conditions) or they don't want to (e.g., due to vaccine hesitancy). We investigate whether non-vaccinators' level of responsibility for not being vaccinated affects individuals' motivation to vaccinate and, thus, to indirectly protect non-vaccinators. In Study 1 (N = 101), the intention to vaccinate increased (Cohen's d = 0.99) when non-vaccinators were described as willing but unable to get vaccinated (low responsibility) compared to when they were able but unwilling to get vaccinated (high responsibility). Study 2 (N = 297) replicated this finding with regard to vaccination behavior in an interactive vaccination (I-Vax) game (OR = 2.38). Additionally, knowing about non-vaccinators' low responsibility also increased the willingness to vaccinate compared to when there was no information on non-vaccinators' level of responsibility. Amplified levels of social welfare concerns in the case of non-vaccinators' low responsibility mediated the latter effect. This finding informs effective communication strategies for improving the vaccination rates.

Objective: Most vaccines provide indirect community protection by preventing the transmission of the disease. Paradoxically, this effect can also motivate omission of vaccination because increasing vaccination rates reduce the risk of infection and, therefore, the individual benefit of vaccination. Consequently, vaccination becomes a social dilemma where individuals' interests conflict with group interests. The current study investigated two social nudge interventions aiming to increasing individuals' motivation to act in the group's interest. Rewarding the attainment of the goal (disease elimination) is hypothesized to increase goal-directed behavior (vaccination). Further, it is assumed that comparisons with another group increase cooperative vaccination within one's own group., Method: In a laboratory experiment, the interactive vaccination (I-Vax) game was used to model the direct and indirect effects of vaccinations. The game was played by 288 participants over 20 rounds. The experimental setup varied the feedback information after each round to implement a 2 (rewarding goal-attainment: present vs. absent) × 2 (intergroup comparison: present vs. absent) between-subjects design., Results: Analyses revealed the expected positive effect of rewarding goal-attainment, which was particularly strong at the beginning and weakened over the course of repeated decisions. The effect of intergroup comparisons was also positive but did not reach conventional criteria of statistical significance., Conclusions: The current experiment shows that communicating and rewarding "small wins" may increase individuals' willingness to act in the group's interest. Intergroup processes deserve further attention and investigation as potential strategies for improving vaccine communication and advocacy. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Annual vaccination is the most effective way to prevent seasonal influenza. However, globally, the recommendations vary from country to country, ranging from universal recommendations, risk-group-specific recommendations, to no recommendation at all. Due to high diversity both in recommendation practice and country-specific preconditions, it is difficult to determine the effect of different recommendations on vaccine uptake. This incentivised laboratory experiment (N = 288) tests the behavioural consequences of different recommendations in a repeated interactive vaccination game. The participants are part of heterogeneous groups, comprised of low- and high-risk type of players. They receive either a universal, risk-group-specific or no recommendation prior to their vaccination decisions. Results show that individuals are sensitive to the recommendations. In detail, a risk-group-specific recommendation increases vaccine uptake of high-risk types. However, at the same time, it decreases vaccine uptake of low-risk types. The results imply that when the proportion of low-risk types in a population is considerably larger than the high-risk group, a risk-group-specific (vs. universal) recommendation comes at the cost of decreased social benefit of vaccination due to the overall lower vaccine uptake. Policy decision-making should therefore complement epidemiological considerations with potential positive and negative behavioural consequences of vaccination recommendations., (Copyright © 2017 John Wiley & Sons, Ltd.)