麻豆传媒

Among children who had an older sibling diagnosed with autism spectrum disorder (ASD), administration of measles-mumps-rubella vaccination made no difference in their subsequent risk of developing ASD, researchers said.

In fact, there was a modest but statistically nonsignificant trend toward lower risk in the vaccinated children with ASD-affected siblings, according to , of The Lewin Group in Falls Church, Va., and colleagues, who calculated an adjusted relative risk of 0.76 (P=0.22) after one MMR dose and 0.56 after two doses (P=0.052), compared with similar unvaccinated children.

For kids with older siblings who did not have ASD, the adjusted relative risk for developing ASD was 0.91 after one dose of MMR, and 1.12 after two doses (both statistically nonsignificant) when compared with children who had not been vaccinated, Jain's group reported in the .

"Taken together, some dozen studies have now shown that the age of onset of ASD does not differ between vaccinated and unvaccinated children, the severity or course of ASD does not differ between vaccinated and unvaccinated children, and now the risk of ASD recurrence in families does not differ between vaccinated and unvaccinated children," , of the University of Washington, wrote in an accompanying editorial.

Jain and colleagues initially identified some 95,000 children born from 2001 to 2007 and with private insurance in the Optum Research Database who had an older sibling.

Jain's team identified children with ASD, children who had an older sibling with ASD, and children with pervasive developmental disorders both specified (e.g., Asperger syndrome) and unspecified.

Overall, 994 (1%) of the children in the study were diagnosed with ASD, and 1,929 (2%) had an older sibling with ASD.

Among those with an older sibling with ASD, 134 (7%) were also diagnosed with ASD; there were 935 index children with ASD-affected older siblings who did not develop the condition themselves.

To control for potential confounders, adjustments were made for preterm birth, vaccine-related allergies, and seizures that could be considered vaccine-related, and childhood chronic conditions (CCC) scores.

Other adjustments were made for parental education level, household income, race/ethnicity, parental ages at the child's birth, mental health benefits, geographical region, and birth year with respect to variations in the child's opportunity to develop or be diagnosed with ASD.

The first vaccine dose was administered at age 2 and the second at age 5. Among the children with affected siblings, vaccination rates were 73% and 86% at 2 and 5 years, respectively.

Kids who had older siblings with ASD were 24% less likely to develop ASD after their first MMR dose, compared with those who were unvaccinated (adj.RR 0.76, 95% CI 0.49-1.18); and the reduction was even greater (44%) after the second dose (adj.RR 0.56, 95% CI 0.31-1.01).

"In general, adjusted 1-dose relative risk estimates were closer to the null than unadjusted estimates, and none of the one-dose relative risk estimates at any age were statistically significant," Jain and colleagues wrote.

The authors suggested that the association between higher rates of autism spectrum disorder in children who were not vaccinated could be a result of selective parental decision making: "parents who notice social or communication delays in their children decide to forestall vaccination," they wrote.

Potential limitations include the possibility that children who lacked insurance claims for the MMR vaccine could have received the vaccine at school or a public health clinic, the positive predictive value of determining a diagnosis of autism spectrum disorder through the claims data was only 87%, and the study design assumed that the siblings were biologically related.

King cited other research that has suggested the origins of ASD might have to do with environmental exposure during early pregnancy, especially in cases of maternal diabetes, or gestational diabetes.

"Controversy seems to follow autism like a tail on a kite," King wrote. But, there might be a positive side to this friction, he wrote: "Controversy can be a stimulus for progress. These studies move the field forward toward a more focused and productive search for temporal and environmental factors that contribute to autism risk."

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