The willingness to vaccinate increases when vaccination protects others who have low responsibility for not being vaccinated.

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.