To the Editor We read with great interest the article by Zerbo et al1 showing that children with autism spectrum disorder (ASD) and their younger siblings have lower rates of vaccination compared with children without ASD and their siblings. As a group of medical professionals and health services researchers working to improve care for children with ASD, we feel understanding vaccination rates is of utmost importance. However, we do suggest an additional factor not discussed in the article that may be contributing to undervaccination in children with ASD: the barriers families face in completing successful medical encounters.