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More Studies Reject Vaccine-Autism Link

News & Comment

Kendrick Frazier

Volume 33.3, May / June 2009

As if more scientific support was needed, a new review of the evidence has again shown no link between vaccines and autism. And a new study from Italy bolsters the case even further.

Concerns by some parents have kept alive the idea of some link, which has not been supported by the scientific literature (see “The Anti-Vaccination Movement,” SI November/December 2007).

Jeffrey S. Gerber and Paul A. Offit of the Division of Infectious Diseases at Children’s Hospital of Philadelphia published a review in the February 15 (2009) Clinical Infectious Diseases (48:456–61) of twenty peer-reviewed scientific studies published between 1999 and 2004. The studies show no connection.

The authors examined three specific claims some have proposed: the combination measles-mumps-rubella vaccine causes autism by damaging the intestinal lining; the mercury-containing preservative thimerosal, formerly in some vaccines, is toxic to the central nervous system; and the simultaneous administration of multiple vaccines overwhelms or weakens the nervous system.

They reviewed the relevant epidemiological evidence and found no support for these claims. In one study, for instance, researchers in England evaluated 498 autistic children born from 1979 through 1992. No change in the rates of autism diagnoses after the 1987 introduction of the MMR vaccine was observed. A study in Denmark compared the incidence of autism in children who had received two different levels of thimerosal or no thimerosal at all. There was no relationship between thimerosal exposure and autism. On the third claim, they note that vaccines “do not overwhelm the immune system . . . even conservative estimates predict the capacity to respond to thousands of vaccines simultaneously.”

“Twenty epidemiologic studies have shown that neither thimerosal nor MMR vaccine causes autism,” conclude Gerber and Offit. “These studies have been performed in several countries by many different investigators who have employed a multitude of epidemiologic and statistical methods. The large size of the studied populations has afforded a level of statistical power sufficient to detect even rare associations.

“These studies, in concert with the biological implausibility that vaccines overwhelm a child’s immune system, have effectively dismissed the notion that vaccines cause autism. Further studies on the cause or causes of autism should focus on more-promising leads.”

The new Italian peer-reviewed study was carried out over a ten-year period and published in the February issue of Pediatrics. Thousands of healthy Italian babies in the early 1990s were given two different amounts of thimerosal as part of their routine vaccinations. Ten years later, 1,403 of those children were identified and given a battery of brain-function tests. Researchers found small differences in only two of twenty-four measurements, and “they might be attributable to chance,” they said. Only one case of autism was found, and that was in the group with the lower thimerosal.

“Put together with the evidence of all the other studies,” said the study’s lead author, Alberto Tozzi of Bambino Gesu Hospital in Rome, “this tells us there is no reason to worry about the effect of thimerosal in vaccines.”

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Kendrick Frazier is editor of the Skeptical Inquirer and a fellow of the American Association for the Advancement of Science. He is editor of several anthologies, including Science Under Siege: Defending Science, Exposing Pseudoscience.