Conclusion
Childhood vaccines do not cause autism. Maternal vaccines have not been shown to cause autism.
The Institute of Medicine (IOM), now called the National Academy of Medicine (NAM), concluded that the body of evidence favors rejection of a causal relationship between autism and MMR vaccine and thimerosal-containing vaccines 1,2. MMR vaccine also prevents rubella disease, thus preventing congenital rubella syndrome and its associated cases of autism.
Why This Is An Issue
Andrew Wakefield, a gastroenterologist at the Royal Free Hospital in England, published a case series in the medical journal The Lancet in 1998. In this article he described 12 children with pervasive developmental disorder associated with gastrointestinal symptoms, 8 of whom had behavioral issues temporally associated with MMR vaccination via retrospective accounts by their parents or physicians 3. Despite study authors acknowledging that this did not prove an association between the vaccine and autism, the lead author went far beyond the paper’s conclusions in a press release and ongoing interactions with the media 4,5. Public concern on the topic grew quickly. In 2010, Dr. Wakefield’s license to practice medicine in the UK was revoked by the British General Medical Council and his study was retracted by The Lancet as evidence of serious professional misconduct mounted. Among other infractions, Wakefield was found to have ordered unnecessary invasive procedures on children without approval of the hospital ethics committee and received undeclared financial considerations from the Legal Aid Board, a group pursuing multiparty legal action for allegedly vaccine-damaged children 6-11. In addition, he had applied for patents for vaccines to rival MMR vaccine. It was also revealed that, for most of the children in the original study, their symptoms either started well before or long after MMR vaccination. Despite the complete refutation of Wakefield’s fraudulent findings by the scientific community, concern still exists among some parents.
Vaccines of Interest
While the initial vaccine targeted by Dr. Wakefield was MMR, the target has shifted over time, especially as epidemiological evidence accumulated that the MMR vaccine was not associated with autism spectrum disorder (ASD). Other targets have included the preservative thimerosal as well as simultaneous vaccination with multiple vaccines. See the Do Vaccine Ingredients Cause Adverse Events? and the Do Combination Vaccines or Simultaneous Vaccination Increase the Risk of Adverse Events? summaries for more details.
Epidemiological evidence
There have been 16 methodologically sound, controlled epidemiological studies exploring an association between ASD and receipt of MMR vaccine 12-20, thimerosal in vaccines 19,21-25, and simultaneous vaccination with multiple vaccines 26,27, in addition to the relevant systematic reviews 2,28-32 and one meta-analysis 33. Together, these studies included more than 1.8 million children. Notwithstanding 11 studies from another pair of authors 34-44, all of which had substantial methodological flaws 2,29,30,45, the epidemiological evidence consistently shows no association between MMR vaccine, thimerosal in vaccines, or simultaneous vaccination and ASD.
A US cohort study suggested a possible increased risk of ASD among children whose mothers received an influenza vaccination during their first trimester of pregnancy, although this association was not statistically significant after a post hoc analysis adjusting for multiple comparisons, and there was no association between ASD and influenza vaccination received during any trimester 46. A Swedish cohort study found no association between pandemic H1N1 influenza vaccination during pregnancy and ASD among children, even when restricting to vaccination in the first trimester 47. Another US cohort study showed that receiving Tdap vaccine during pregnancy is not associated with increased risk of ASD in the child 48.
Proposed biological mechanism
The overlapping times of childhood vaccine administration and usual onset of ASD symptoms have led to speculations about a possible causal pathway; however, the proposed links have been unsubstantiated 49. Several different theories were proposed to attribute the cause of ASD to vaccines. Wakefield suggested that a dysregulated immune response to measles antigen in the MMR vaccine led to persistent intestinal infection, allowing “toxins” to enter the blood stream and enter the central nervous system leading to developmental regression in children. He claimed support for this because of his alleged detection of measles virus RNA in bowel specimens of several children with ASD 3. However, his referenced study was found to be fraudulent, and no evidence of persistent infection has been shown in studies that used appropriate methods 50-52. Another proposed trigger for ASD was thimerosal, an ethyl-mercury containing preservative that used to be present in some vaccines, although not in the MMR vaccine. This theory was based on observed similarities in some features of ASD and mercury poisoning 53; however, the degree of these similarities and the plausibility of this suspected association was refuted by neurologists 54. The IOM found no valid mechanistic evidence connecting MMR or thimerosal-containing vaccines and ASD 1,2.
References
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