The uptake of routine childhood vaccinations has declined globally since the start of the COVID-19 pandemic, due in part to increased vaccine hesitancy among parents. The Moral Foundations Theory proposes six foundations which can be targeted to increase vaccine uptake. In this study, we tested whether a post by UNICEF with a purity violation message could affect vaccine attitudes among parents in Argentina, where routine immunization coverage has been declining since 2014. Using an online randomized controlled survey experiment, we included 1,511 parents with a child under 12 years in Argentina. Outcomes were measured on the Vaccine Trust Indicator (VTI). We found that the purity violation message significantly affected vaccine attitudes: 80% of the intervention group scored high on the Vaccine Trust Indicator, compared to 73% in the control group (coefficient: 0.33, 95% Confidence Interval 0.20-0.47). Vaccine messaging framed as a purity violation is a promising way to improve vaccine attitudes among parents.