Autism spectrum disorder (ASD) is a neurological condition considered a developmental disorder that appears in the first two years of life. Clinically it can be diagnosed by around 18 months, but is often not formally diagnosed until later. ASD is typified by difficulty with communication and social interaction, restricted range of interests, and repetitive behaviors. It is considered a spectrum because there is a wide variation in symptoms.

ASD has been caught up in a controversy because of false and now thoroughly debunked claims that it may be caused by vaccination, such as the MMR vaccine. The CDC recommends all children get their first dose of MMR at 12-15 months, and a second at 4-6 years old. The correlation between the timing of the first dose and the onset of recognizable symptoms of ASD likely lead some parents to falsely conclude that the two were causally related, feeding the myth that the MMR vaccine causes autism.

The pre-existing anti-vaccine movement immediately focused on this claim, which was given a huge boost from a fraudulent and now retracted study by the infamous Andrew Wakefield. The MMR-autism “hypothesis” (which is being generous) was soon thoroughly discredited by scientific evidence. The anti-vaccine movement, not deterred by facts, morphed their claims to focus on mercury-containing thimerosal (which the MMR vaccine does not contain). This claim died at the slings of scientific evidence as well. Most dramatically, thimerosal was removed from the standard vaccine schedule in 2001, with any lingering doses used or discarded certainly by 2002 or 2003. The anti-vaccine movement predicted this would precipitate a dramatic drop in autism diagnoses. Scientists predicted it would not. It didn’t. Diagnostic rates continued to rise in the same pattern, likely caused by expanded diagnosis and surveillance. The thimerosal hypothesis was dead.

The anti-vaccine movement, undeterred by evidence, morphed their claims into simply “vaccines” were the cause of autism, without specifying a specific vaccine or ingredient. This gives them the ability to maintain their anti-vaccine claims, while constantly evading new evidence.

However, there is another scientific angle from which to approach the question of vaccines and autism – what is the true cause of ASD, and when does it truly first appear? Over the last few decades, despite the false controversy over vaccines, researchers have been learning a great deal about the genetic causes of ASD. There are many genes now associated with ASD, genes that code for brain-specific proteins. This does not rule out an environmental influence, but at its core ASD appears to be a multifactorial genetic disorder.

Further, there is the question of when ASD is first manifest. Formal clinical diagnosis has to wait until children are old enough to clearly display behavioral signs of ASD, which again begins around 18 months. If ASD is mostly or entirely genetic and developmental, however, then there should be more subtle signs of the disorder at a younger age. As soon as researchers started looking, they started to find changes in behavior and head circumference detectable between 6-12 months, 8-12 months, and even 4-6 months in one study. Of course, if ASD is present at 4-6 months, then later vaccines cannot be the cause.

If ASD is mostly genetic, however, evidence should go back even further, to the womb. The first such evidence was published in the NEJM in 2014. Researchers examined the brains of children with ASD who died. The autopsies revealed patches of disruption in the normal 6-layer structure of the cortex. These layers form early in development, and the changes seen had to occur during that phase of brain development, which puts the onset of ASD in early fetal development.

There is also more direct evidence. A study from January 2022 found an association between ultrasonography fetal anomalies (UFAs) and a later diagnosis of ASD, compared to their neurotypical siblings and age-matched general population controls. Further, the number of UFAs correlated with the severity of symptoms of ASD.

A more recent study used MRI scans to examine the brains in utero of children with and without a later clinical diagnosis of ASD. This study allowed for a more detailed examination of fetal brains than ultrasound. They found:

The insula/insular lobe showed statistically significantly larger volume in ASD than that in all three control groups in the lobar comparison. In the regional comparison, the ASD group had statistically significantly larger amygdala, hippocampal commissure, and insula compared to the non-ASD controls with neurological and non-neurological comorbidities.

These finding correlate with features of the ASD brain in adults, so this is likely not a random fluke.

We now have multiple studies that convincingly demonstrate that ASD is a neurodevelopmental disorder that manifests mostly in late gestation. There are strong genetic correlates, but the environment of the womb appears to also play a role. However, this consensus of evidence is incompatible with the notion that childhood vaccines play any role in ASD.

Author

  • Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.

Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.