evidence: In the present study, within each recommended age category of vaccination before age 10 years, younger siblings of children with ASD were significantly more likely to be undervaccinated compared with younger siblings of children without ASD, suggesting that the ASD diagnosis of the older sibling may have contributed to the undervaccination of the younger children. Knowing that younger siblings of children with ASD are at higher risk for ASD34-37 may have led some parents of children with ASD to either delay or refuse vaccinations for the younger siblings. It is also possible that health care professionals may have been more likely to assign the code for vaccine refusal for the siblings of children with ASD when they know that the family has a child with ASD. Parents of children with ASD may also delay or alter the vaccination schedule of their younger children because of concerns that vaccines may have had a role in causing the ASD of the older siblings27,30; this is despite considerable scientific evidence that vaccines do not cause autism. [...] F. MMR vaccine and autism: a review of the evidence for a causal association. [...] CP, et al. Autism and measles, mumps, and rubella vaccine: no epidemiological evidence for a causal association.