Childhood vaccines do not cause autism, according to a comprehensive body of evidence that includes dozens of controlled epidemiological studies, multiple systematic reviews, and a meta-analysis covering more than 1.8 million children. The Institute of Medicine, now the National Academy of Medicine, has concluded that the evidence favors rejection of any causal relationship between autism spectrum disorder and either the MMR vaccine or thimerosal-containing vaccines.
The findings directly address one of the most persistent and consequential vaccine safety questions of the past three decades. Maternal vaccines have likewise not been shown to cause autism. The MMR vaccine also prevents rubella disease, thereby preventing congenital rubella syndrome and its associated cases of autism — a point that underscores the protective role of immunization.
The origin of the controversy traces to 1998, when Andrew Wakefield, then a gastroenterologist at the Royal Free Hospital in England, published a case series in the medical journal The Lancet. The paper described 12 children with pervasive developmental disorder and gastrointestinal symptoms, 8 of whom had behavioral issues that parents or physicians retrospectively linked in time to MMR vaccination. Although the study authors acknowledged that their work did not prove an association between the vaccine and autism, Wakefield went far beyond the paper’s conclusions in a press release and in ongoing media interactions, and public concern grew rapidly.
In 2010, Wakefield’s license to practice medicine in the United Kingdom was revoked by the British General Medical Council, and The Lancet retracted the study as evidence of serious professional misconduct accumulated. Among other findings, Wakefield was determined to have ordered unnecessary invasive procedures on children without approval from the hospital ethics committee and to have received undeclared financial considerations from the Legal Aid Board, a group pursuing multiparty legal action on behalf of children alleged to have been damaged by vaccines. He had also applied for patents on vaccines intended to rival the MMR vaccine. It was further revealed that for most of the children in the original study, symptoms began either well before or long after MMR vaccination.
Despite the complete refutation of Wakefield’s findings by the scientific community, concern persists among some parents. The target of that concern has shifted over time. While the initial focus was the MMR vaccine, attention later moved to thimerosal, an ethyl-mercury-containing preservative once used in some vaccines but never in the MMR vaccine, and to the practice of giving multiple vaccines simultaneously. Each of these hypotheses has been tested extensively.
The epidemiological record includes numerous methodologically sound, controlled studies examining associations between autism spectrum disorder and MMR vaccination, thimerosal exposure, and simultaneous administration of multiple vaccines, alongside relevant systematic reviews and one meta-analysis. Together these studies encompass more than 1.8 million children. With the exception of 11 studies by another pair of authors, all of which contained substantial methodological flaws, the evidence consistently shows no association between any of these exposures and autism spectrum disorder.
Research on maternal vaccination has produced similarly reassuring results. One United States cohort study suggested a possible increased risk of autism among children whose mothers received an influenza vaccination during the first trimester of pregnancy, but the association was not statistically significant after a post hoc analysis adjusting for multiple comparisons, and no association was found with influenza vaccination during any trimester. A Swedish cohort study found no association between pandemic H1N1 influenza vaccination during pregnancy and autism in children, even when restricting the analysis to first-trimester vaccination. Another US cohort study showed that receiving the Tdap vaccine during pregnancy is not associated with an increased risk of autism in the child.
Proposed biological mechanisms have also been examined and unsubstantiated. The overlapping timing of childhood vaccine administration and the usual onset of autism symptoms has fueled speculation about a causal pathway, but the proposed links have not held up. Wakefield suggested that a dysregulated immune response to measles antigen in the MMR vaccine led to persistent intestinal infection, allowing toxins to enter the bloodstream and central nervous system and causing developmental regression. He claimed support from alleged detection of measles virus RNA in bowel specimens from several children with autism. However, his referenced study was found to be fraudulent, and studies using appropriate methods have shown no evidence of persistent infection.
Another proposed trigger, thimerosal, was theorized to cause autism based on observed similarities between some features of autism and mercury poisoning. Neurologists refuted both the degree of those similarities and the plausibility of the suspected association. The Institute of Medicine found no valid mechanistic evidence connecting MMR or thimerosal-containing vaccines to autism spectrum disorder.
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