The Vaccine-Autism Link: Facts and Myths

June 11, 2007 — -- A federal court begins hearings today on the more than 4,800 claims by parents of autistic children that the mercury-containing preservative thimerosal, present in certain routine vaccinations, led to their children's disorders.

It is the day that many of these families have pushed for since 1999.

Concerns about thimerosal's possible link to autism have gained publicity in recent years as the occurrence of autism in children has climbed steadily in the United States. Doctors have struggled to find a clinical reason for the increase. Because vaccinations are mandatory for all children at a certain age, some parents and doctors believe the vaccines indicate a common link that cannot be ignored.

However, current research weighs heavily against the premise of a thimerosal-autism link, as thus far a number of large scientific studies have shown no association between the two.

Most recently, a review of past research published this month in the journal Current Opinion in Infectious Diseases suggests "increasingly convincing epidemiologic and laboratory evidence against a causal relation of several alleged adverse events following immunization."

The findings bolster the position that has been steadfastly held by the Institute of Medicine (IOM) that no link can be established between thimerosal and autism.

Researchers Speak Out

Prior to the summary, most physicians and researchers have agreed that the suggestion of such a link is likely baseless, as seen in their previous statements on the issue (responses collected from telephone interviews conducted in June 2005):

"Current literature demonstrates very clearly that there is no relationship between thimerosal in vaccines and autism," noted Dr. Julia McMillan of the American Academy of Pediatrics' Committee for Infectious Diseases. "Thimerosal has been studied more than anything else, and it is distracting a very important effort to get at the real cause of autism."

"It doesn't make biological sense," said Dr. Paul Offit, a member of the Advisory Committee on Immunization Practices to the Centers for Disease Control and Prevention. "Fourteen separate studies showed that there was never an increased risk [of autism from thimerosal], but it is very hard to unring that bell."

Dr. Eric Hollander, director of the Seaver and New York Autism Center of Excellence, concurs. "There are no links between vaccines, thimerosal, or ethyl mercury and autism," he said. "Some people have unusual immune responses to vaccines, but that cannot account for autism."

The issue may be a moot point. Despite the lack of scientific evidence of a link between thimerosal and autism, a number of public health organizations and vaccine manufacturers agreed in July to reduce or eliminate thimerosal from vaccines as a precautionary measure. As a result, none of the vaccines used in the United States to protect preschool children against 12 infectious diseases contain thimerosal as a preservative (with the exception of some influenza vaccines), according to the CDC.

Some researchers say that the absence of a link has already been demonstrated in a real-world setting in Denmark, where thimerosal was removed from all vaccines in 1992. Despite this move, autism rates in the country continued to rise until 2004, the year in which the Danish study was terminated.

Vaccine Avoidance: a Greater Danger?

But some said the suggestion that vaccines could be to blame for autism may already be having an impact on public health, as parents concerned about the possibility of dangers may be less inclined to have their children immunized:

"The bottom line is that vaccines prevent life-threatening and life-risking diseases," AAP's McMillan said. "It is about the most effective public health measure we have. It is very important that we do not allow misinformation to affect that."

"In the case of the MMR [measles-mumps-rubella] vaccine, we know that it prevents diseases that can injure and kill children," said Dr. Gary Freed, director of general pediatrics for the University of Michigan Health System.

"Especially in the U.K. and Ireland, home to the early proponents of a connection between MMR and autism, decreased immunization rates have resulted in needless epidemics of measles which have killed several children."