Autism and MMR Vaccine: An Overview

In the past few years, there has been public interest in a theory that suggested a link between the use of thimerosal, a mercury-based preservative used in the measles-mumps-rubella (MMR) vaccine, and autism. Although mercury is no longer found in childhood vaccines in the United States, some parents still have concerns about vaccinations.

 

Many well-done, large-scale studies have now been done that have failed to show a link between thimerosal and autism. A panel from the Institute of Medicine is now examining these studies, including a large Danish study that concluded that there was no causal relationship between childhood vaccination using thimerosal-containing vaccines and the development of an autism spectrum disorder, and a US study looking at exposure to mercury, lead, and other heavy metals.

 

Autism and MMR Vaccine: The Wakefield Study

The study that initially suggested a link between the autism and the MMR vaccine was published in Lancet by Wakefield and colleagues in 1998. Based on data from 12 patients who were diagnosed with bowel disease and autism, the authors of this study speculated that, based on the parents' recall, the measles/mumps/rubella (MMR) vaccine may have been a possible cause of the neurological disease and bowel problems. The researchers theorized that the bowel disease could have led to decreased absorption of essential vitamins and nutrients, which in turn could have led to developmental disorders, like autism. However, the study focused on anecdotal evidence; the authors performed no scientific analyses to substantiate their theory.

 

An editorial published in the same issue of Lancet voiced concerns about the validity of the study. It is difficult to determine whether the 12 reported cases represent an unusual or unique clinical syndrome without knowing the size of the patient population from which the 12 were identified and the duration of time allotted for reporting cases. Without evidence supporting or disputing a selective referral process, a referral bias that sent only the 12 cases noted in the study to the authors practice cannot be ruled out. The authors stated that additional cases have been identified; however, detailed clinical characterization of the neurological diseases and documentation of timing of vaccination, autism, and bowel disease onset in all these children has not been provided.

 

The theory that autism might be caused by poor absorption of nutrients resulting from a bowel inflammation is also not supported by clinical data. In at least four of the 12 cases reported in the Lancet study, behavioral problems appeared before the onset of symptoms of inflammatory bowel disease; that is, the effect preceded the proposed cause. The same authors published another study in which they performed highly specific laboratory assays of patients with inflammatory bowel disease following MMR vaccination, the cause for autism they posited in their initial study. These patients tested negative for the measles virus, which would not support the theory the authors posed in their original study.

 

Subsequently, these authors and other collaborators have reported detection of measles virus from blood cells and intestinal samples from children with autism and inflammatory bowel disease. The authors also report detection of wild type and vaccine strains of measles virus RNA in patients with inflammatory bowel diseases without the diagnosis of autism, thus making it impossible to associate measles virus RNA with autistic disorders. Other scientists have published studies that measles virus RNA and protein are not present in intestinal specimens from patients with inflammatory bowel diseases.

 

Although gastrointestinal problems have been documented in a subgroup of people with autism, studies remain inconclusive. One recent study indicated that unrecognized gastrointestinal disorders in nonverbal autistic patients may contribute to behavioral problems. However, authors of the study were pointed in their conclusion that additional research is required to determine any possible association between the brain and gastrointestinal dysfunction in autistic children.