Vaccination Rates among Younger Siblings of Children with Autism.

To the Editor: Recent outbreaks of illnesses that may be prevented by vaccines have increased public debate about vaccination behaviors (i.e., whether or not and when parents choose to vaccinate a child), and California has been a key affected state. One reason that parents choose not to have their children vaccinated is the perceived link between vaccines and autism spectrum disorder. This reason is particularly relevant for parents of a child who has autism spectrum disorder, since concerns that the disorder will develop in subsequent children may be more pronounced. Since 2009, we have tracked the development of infants who have a full biologic older sibling with a diagnosis of autism spectrum disorder. The risk of this disorder is increased by a factor of approximately 20 among these “high-risk” infants.1 Our comparisons of such infants with age-matched “low-risk” infants who have an older sibling without autism spectrum disorder allows for the identification of putative biomarkers of this disorder.2 Although vaccination behaviors were not our primary focus, studies showing decreased rates of vaccination against measles–mumps–rubella (MMR) among high-risk infants prompted us to analyze available data on vaccination behaviors in our sample of 206 families from southern California (71 of which had a child with autism spectrum disorder and 135 that did not have a child with autism spectrum disorder) (Table 1).3,4 Childhood vaccination and autism spectrum disorder continue to be linked in the minds of many people despite overwhelming evidence to the contrary.5 Adding to this evidence, our data showed no significant difference between rates of vaccination among children with and those without autism (100.0% vs. 98.5%; P = 0.30). However, there were two additional interesting findings. Families with children who had autism spectrum disorder were less likely to vaccinate subsequent children. Specifically, the rate of vaccination among full biologic infant siblings of children with autism spectrum disorder was 83.1%, as compared with 97.0% among low-risk infants (Pearson chi-square value with one degree of freedom, 12.62; P<0.001). These findings are consistent with reported rates of MMR vaccination among children at older ages and across broader sampling regions.3,4 Our results also suggest that changes in vaccination behavior may relate to adverse reactions to vaccine. In particular, parents who had an older child with autism spectrum disorder retrospectively reported a higher rate of adverse reactions to vaccination among the older child than did those who did not have an older child with autism (22.6% vs. 3.8%; Pearson chi-square value with one degree of freedom, 16.87; P<0.001). Likewise, parents who had an older child with autism retrospectively reported a higher rate of these reactions among the infant sibling than