Expert Q+A: Is There a Link Between Vaccines and Autism?

Q. What causes autism?

The million-dollar question. There appear to be four chief suspects:

  1. Genetics. We know genetics plays a role. Studying twins is an obvious way to detect genetic disorders. If one identical twin has autism, up to 96 percent of the time, so will the other twin. And siblings of ASD kids have a 5 percent risk of having an autistic disorder.6 To date, the exact gene has not been identified, but it may reside on the X chromosome, which may explain the prevalence of autism in boys.7 In fact, there is a genetic syndrome (called Fragile X) that is one known cause of autism.

    In 2008, researchers identified a specific gene in some kids with autism. This gene is involved in controlling brain cell communication.8 It appears that some kind of mutation in this gene causes a risk of autism within families. Other researchers have found abnormalities on chromosomes of autistic kids. Hence it appears that autism is caused by several different genetic defects, although researchers haven't quite figured out the puzzle yet.9

    One study has shown that dads over the age of 40 have SIX times greater risk of having a child with an autistic disorder than dads who are younger than 30.10 Hence, autism has eerie echoes of Down Syndrome, a genetic defect that is more common when a mother has "advanced maternal age" (over age 35). All of these studies show that genetic defects are a strong suspect in autism.
  2. Abnormal brain growth. Although the cause is unknown, autistic children have problems with brain growth. Babies are born with immature brains that grow rapidly and make nerve connections called synapses, like an information superhighway. In the normally growing brain, some branches of this superhighway get "pruned." In the autistic child's brain, the pruning process is defective. This may explain why babies with autism have abnormally rapid head growth under one year of age. Boys with ASD seem to have higher levels of hormones (insulin-like growth factors), which may contribute to the larger head size, weight, and body mass index.11
  3. Environmental trigger. Is there some environmental exposure that sets off abnormal brain development in a genetically predisposed baby? Maybe. And that exposure may happen at or shortly after conception--before a mother even knows she is pregnant. There is a critical period of fetal brain development that occurs at 20-24 days after conception where the brain is most sensitive to injury.

    Here are just a few theories that scientists are exploring as a cause for autism: flu exposure during pregnancy, and folic acid levels in Dad-to-be's sperm (possibly a too-high level can lead to problems). Studies done by the Environmental Working Group have found about 280 environmental toxins in umbilical cord blood--could one of these be a trigger? There is also a growing body of evidence that newborns who are later diagnosed with ASD already have abnormal levels of certain proteins in their brains. So, having an environmental trigger in the womb during a critical period of brain development seems a plausible explanation for autism.

    What about vaccines? There has been much talk about this theory, specifically that trace amounts of mercury used as a preservative in many vaccines prior to 2001 caused a spike in autism. We discussed this issue in depth in Baby 411, but just to sum up: the scientific evidence does not support this theory. Research during the past ten years has taken a long hard look at vaccines and found conclusive evidence that vaccine exposure is NOT the turn-on switch for autism.12 And no, despite what you might read online from fringe groups or plaintiff lawyers, there is no conspiracy among pharmaceutical companies to inflict autism on unsuspecting children.

    The Centers for Disease Control have long-term studies underway to examine vaccines and autism. The most recent results, published in the New England Journal of Medicine, showed that the mercury preservative previously present in vaccines had no significant effect on either intelligence or developmental delays in kids ages seven to 10. The results of the CDC's study on mercury preservative and autism specifically will be published after this booklet goes to print. Stay tuned on our website ( for updates.
  4. Premature birth. A recent study in the journal Pediatrics found that premature babies born at 25 to 26 weeks gestation have a 25 percent chance of developing an autism spectrum disorder.

    Bottom line: Researchers don't know what causes autism, although the above factors provide clues. The goal is to find a way to PREVENT autism . . . but we aren't there yet.

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