Supplementary Material

A1 Background Vaccine hesitancy, the decision to reject or delay accepting vaccines despite the availability of vaccine services (MacDonald, 2015), is an urgent problem in combating the Covid-19 pandemic because in countries with sufficiently available doses, vaccine uptake is slowing. Coming closer to herd immunity (Anderson & May, 1985) will be a prerequisite of reducing non-pharmaceutical interventions such as social distancing, school closures, or mask wearing. The moral dimensions of liberty, purity, authority, and possibly harm have been found to correlate with vaccination hesitancy measured as parents’ and caregivers’ attitudes and intentions towards vaccina-tion against child’s diseases (Amin et al., 2017; Rossen et al., 2019). Further predictors of vaccine hesitancy have been demonstrated to be vaccine attitudes, political atti-tudes, conspiracy beliefs, faith in science, and cognitive thinking style. All these predictors are known to be interrelated and have never been investigated together in a single study. A2 Objectives and Research questions The present study aims at investigating the relationship between moral values with psychological pre-cursors to adults’ decisions to be vaccinated against Covid-19, a decision directly concerning the de-ciders themselves. First, from the perspective of applied research, vaccination programs need to be informed about psy-chological factors beyond beliefs in safety and efficacy of vaccines in order to tailor messaging di-rected especially at vaccine hesitant subpopulations. The objective of the present study is to identify subpopulations in vaccine hesitant persons and their moral signatures. The moral signatures of vac-cine programs have been demonstrated to have an effect on vaccine uptake (Heine & Wolters, 2021). Second, from the perspective of basic research, the present study investigates the relationship be-tween vaccine beliefs, vaccination decisions, and moral values. The current pandemic situation allows to differentiate vaccine attitudes and mere intentions from the factual decision to become vaccinated or not. This allows to answer the research question: Do moral values predict the decision to vaccinate against Covid-19 over and above vaccination beliefs and attitudes? Third, the study aims at a direct replication of a previously unpublished finding of the relationship between moral foundations and the decision to vaccinate against Covid-19. A3 Participants A sample of German adults is aimed to be drawn with 50% participants already vaccinated against Covid-19 at least once and 50% who do not intend to be vaccinated. A4 Study method Participants are asked to complete an online survey using the moral foundations questionnaire (J. Graham et al., 2011), psychological antecedents of vaccination in general and against Covid-19 spe-cifically („5C model“ Betsch et al., 2020), conspiracy mentality (Stojanov & Halberstadt, 2019), faith in science (Farias et al., 2013), propensity to think rationally (Thomson & Oppenheimer, 2016), open-minded thinking style about evidence , and political attitudes (Akkerman et al., 2014; Decker et al., 2013). First, data-driven clustering methods will be used to model groups of vaccination hesitancy and their relationship to the endorsement of moral domains. A5 Open Science The analysis is pre-registered in this file. Study materials are available in the second file of this pre-registration. Data and executable analysis code will be made publicly available.

The moral dimensions of liberty, purity, authority, and possibly harm have been found to correlate with vaccination hesitancy measured as parents’ and caregivers’ attitudes and intentions towards vaccination against child’s diseases. The present study investigates the relationship between moral dimensions and the intention/decision to vaccinate against Covid-19 in adults, rather than specifically parents. The decision to vaccinate against Covid-19 concerns the deciders themselves rather than another person – their wards. German adults recruited from social media groups with vaccination, health, and Covid related topics (such assupporting or rejecting vaccination in general or against Covid-19 specifically) will be investigated. Participants are asked to complete an online survey using the moral foundations questionnaire (J. Graham et al., 2011), general vaccination hestitancy (Rossen et al., 2019), attitudes towards vaccination against Covid-19, and political orientation on a left-right spectrum as well as political party association. Bayesian multiple linear and multinomial logistic regressions as well as data-driven clustering methods will be used to model groups of attitudes and their relationship to the endorsement of moral domains.