Background: When Germany introduced a measles vaccine mandate in 2020, it was accompanied by worries about potential unintended consequences. Drawing upon Psychological Reactance Theory, experts worried the mandate might lead to opposition and refusal of other voluntary vaccines. However, following System justification theory, some have argued that reactance to a policy is only temporary and will dissipate over time. We examined how reactance and attitude towards the mandate evolved, whether reactance prompted attempts of freedom restoration, and the role of trust in mandate acceptance.


Methods: We conducted a preregistered longitudinal online survey with an initial sample of N=6901 respondents, followed at six waves over 20 months. The sample included parents subject to the mandate and adults without children as a comparison group. Using linear mixed-effects models we analyzed changes in reactance and attitudes over time, group differences, and trust as a determinant. Logistic regression assessed whether reactance predicted uptake of another voluntary vaccine.


Results: Reactance levels were low but stable across groups, while attitudes towards the policy became more negative over time (β=-0.01, p<.001). Parents exhibited higher reactance than adults without children (β=-0.12, p<.001), but attitudes toward the mandate did not differ between groups. Higher trust correlated with lower reactance (β=-0.28, p<.001) and more positive attitudes (β=0.43, p<.001). Notably, higher initial reactance among parents was linked to lower likelihood of vaccinating their children with another voluntary vaccine after 20 months (OR 0.49, p<.001). Exploratory analysis, conducted to contextualize results, revealed gaps in the mandate's implementation.


Conclusions: Contrary to System justification theory, reactance to a policy might not simply dissipate over time, and policy makers should be mindful of individuals taking their freedom back on other voluntary vaccines. Trust can keep the anger at bay and help justifying the policy. Effective enforcement can aid in rationalizing vaccine mandates and mitigating opposition.