This paper studies non-adherence in the Danish Childhood Vaccination Program using a nationwide introduction of a vaccination reminder letter policy and administrative data from 2011–2017. First, I provide causal estimates of how the reminder letter policy affects vaccination adherence using a Regression Discontinuity Design (RDD). Second, I link parental responses to the reminder letter to parents’ causes for being non-adherent. I find that the reminder letter policy positively affects adherence. However, 72% of non-adherent parents are non-responsive to the reminder letter indicating that reluctance and not inattention is the leading cause for non-adherence. Thus, other policies beyond reminder letters – such as mandatory vaccination laws – are necessary to substantially increase vaccination coverage.