IOM finds no evidence linking MMR vaccine and thimerosal to autism

June 2004

In what may be its strongest statement to date, the Institute of Medicine (IOM) issued a report recently that said there is no evidence that thimerosal in vaccines or the measles-mumps-rubella vaccine (MMR, M-M-R-II, Merck), causes autism.

The committee concluded that the evidence rejects the idea that MMR vaccine or thimerosal-containing vaccines cause autism. The committee also said that the potential biological mechanisms for vaccine-induced autism were only theoretical.

In a press release, the AAP called the report pivotal and said it was a “significant affirmation of vaccine safety.”

The committee did not recommend a policy review of the current vaccination schedule, but it did recommend a public health response that supports vaccine safety. “Immunization to protect children and adults from infectious diseases is one of the greatest achievements of public health,” the report said.

“The Institute of Medicine statement is a reaffirmation of science and a mandate for stringent review of data and publication materials,” said Richard F. Jacobs, MD, associate medical editor of Infectious Diseases in Children.

Wakefield was wrong

This eighth and final report of the IOM’s Immunization Safety Review Committee reviewed recent studies that examined the hypothesis that the MMR and thimerosal-containing vaccines are causally associated with autism. Although the committee issued two other reports on the issue, it was asked to review the latest published and unpublished epidemiological studies regarding causality and studies of potential biologic mechanisms by which these immunizations might cause autism.

One of the primary concerns about MMR vaccine and thimerosal-containing vaccines was that they might cause autistic spectrum disorders, which are incurable conditions that are associated with serious developmental problems in children. The concerns came to light in 1998 after a study by Andrew Wakefield, MD, and colleagues described 12 children with bowel symptoms and developmental regression, including autistic spectrum disorders, shortly after receipt of MMR vaccine. Unfortunately, autistic symptoms typically do not become evident until the second year of a child’s life, which is about the time that children receive MMR vaccine, so the association is probably coincidental, many experts have said.

The Wakefield study was recently retracted by The Lancet amid allegations of selection bias and ethics, when it became known that Wakefield allegedly received funding from the Legal Aid Board, a British legal fund representing parents of children suing the vaccine’s maker over alleged injuries.

The IOM committee said Wakefield’s study did not prove an association and subsequent studies could not confirm Wakefield’s hypothesis. These studies, including three controlled observational studies and two uncontrolled observational studies, consistently provided evidence of no association, the report said.

“The nonscientific public and political firestorm that was created by the publication of nonscience and biased information from a person with obvious conflicts of interest that initiated this issue has now been answered,” said Jacobs, who is also the Horace C. Cabe professor of pediatrics at the University of Arkansas for Medical Sciences and chief of pediatric infectious diseases at Arkansas Children’s Hospital in Little Rock. “The IOM’s finding of no causality is so strong that it does not recommend a policy review of the current schedule for the administration of MMR vaccine. This, coupled with the retraction of the original publication by most of the authors, is yet another victory for science and the scientific process.”

Another firestorm was started in 1999, when the AAP and the Public Health Service called for vaccine manufacturers to remove thimerosal from vaccines because they wanted to reduce, where possible, a child’s exposure to all sources of heavy metals, including mercury. Although there was never evidence of toxicity from the use of the ethyl mercury–containing preservative, thimerosal, used in some childhood vaccines, reducing child exposure to mercury in any form was considered important.

Today, vaccines routinely given to young children are either completely free of thimerosal or have only trace amounts, the AAP said in its release.

To prove or disprove is not the question

Paul Offit, MD, a member of the editorial advisory board of Infectious Diseases in Children, called the report “a pleasant surprise.” He said the IOM was willing to make definitive statements that there is no connection between MMR and autism or thimerosal and autism. “Essentially, they said evidence rejects the causal hypothesis that MMR causes autism; the same can be said for thimerosal as a cause of autism,” he said.

He said the take-home message for pediatricians and parents should be reassuring: that the vaccine is safe for most children, and that the current immunization policy should not be changed.

“I think what the IOM did is it reviewed the strengths and weaknesses of the scientific data and has concluded what everyone who has looked at those data have also concluded: that parents can be reassured, that doctors can be reassured, neither MMR nor thimerosal causes autism,” added Offit, who is chief of infectious diseases at Children’s Hospital of Philadelphia.

In addition, the committee recommended that it was time to take the money being spent trying to prove or disprove this hypothesis and spend it on autism research that can get a better return. “Research should be directed toward areas that are more promising, which is to say, not vaccines,” Offit said.

A statement by the American Medical Association agrees, saying that the “finding will allow research funding to be appropriately channeled into the investigation of the actual causes of autism.”

The sentiment was also voiced by the AAP. “For most parents, today’s report should assure them of the safety of vaccines,” said AAP President Carden Johnston, MD, in a statement. “There’s no doubt we must find the causes of autism, but we need to target other more promising research areas. The academy supports aggressive research into the causes, treatment and prevention of autism.”

IOM report echoes other data

The IOM report can be added to the numerous scientific reviews and studies that came to the same conclusion: neither MMR vaccine nor thimerosal-containing vaccines cause autism, the AAP said.

“Parents shouldn’t kid themselves. Diseases like measles haven’t disappeared,” Johnston said. “They are kept at bay through widespread vaccination. At any time, an unvaccinated child could contract one of these dangerous and life-threatening diseases. This report will help parents make an informed decision to fully immunize their children.”

Before the measles vaccine was introduced in the United States, there were 3 to 4 million cases of measles each year, and an average of 450 deaths from complications. More than 50% of Americans had measles by the time they were 6 years old, and 90% had the disease by the time they were 15, according to the CDC. After the vaccine was introduced, measles cases dropped by 98%.

“The opportunity now is to turn this statement into improved immunization coverage rates in infants and children. This requires an educational and organization commitment by professional societies and individual physicians,” said Jacobs.

For more information:

• Immunization Safety Review Committee, Board on Health Promotion and Disease Prevention. “Immunization Safety Review: Vaccines and Autism.” 2004. Washington, D.C. National Academy Press.
• Murch SH, Anthony A. Casson DH, et. al. Retraction of an interpretation. Lancet. 2004;363(9
• Wakefield AJ, et al. Ileal-lymphoid-nodular hyperplasia, non-specific colitis and pervasive developmental disorder in children. Lancet. 1998:351:637-641.
• View the IOM report online at www.iom.edu/report.asp?id=20155.