Myths tend to be persistent and require periodic maintenance debunking. The anti-vaccine movement arguably can credit its recent increase in effect to successfully spreading fears that vaccines in general, and particularly either the MMR vaccine (mumps, measles, and rubella) or the vaccine preservative thimerosal, are linked to autism. This claim was never based on legitimate science, and over the last 15 years has been overwhelmingly repudiated by multiple independent lines of scientific evidence.

It is easier to spread fears than it is to reassure anxious parents with abstract scientific data, but still we must try. It also seems that giving people information is not an effective way to change their opinions or their behavior. But at the very least I hope to better inform those who are already on board with the science-based approach, and perhaps we can reach the occasional person on the fence who is simply misinformed and open to changing their mind.

A new meta-analysis seems like a good opportunity to remind the public that vaccines are safe and effective, and that they are not linked to neurodevelopmental disorders. The study is: “Vaccines are not associated with autism: An evidence-based meta-analysis of case-control and cohort studies,” by Taylor, Swerdfeger and Eslick, is published in the journal Vaccine. They found:

Five cohort studies involving 1,256,407 children, and five case-control studies involving 9920 children were included in this analysis. The cohort data revealed no relationship between vaccination and autism (OR: 0.99; 95% CI: 0.92 to 1.06) or ASD (OR: 0.91; 95% CI: 0.68 to 1.20), or MMR (OR: 0.84; 95% CI: 0.70 to 1.01), or thimerosal (OR: 1.00; 95% CI: 0.77 to 1.31), or mercury (Hg) (OR: 1.00; 95% CI: 0.93 to 1.07). Similarly the case-control data found no evidence for increased risk of developing autism or ASD following MMR, Hg, or thimerosal exposure when grouped by condition (OR: 0.90, 95% CI: 0.83 to 0.98; p = 0.02) or grouped by exposure type (OR: 0.85, 95% CI: 0.76 to 0.95; p = 0.01).

Of course, it is impossible to prove a negative, but those p-values, confidence intervals, and odds ratios are all very reassuring. This is, collectively (and that’s the point of a meta-analysis), a large body of evidence showing a complete lack of association between autism and ASD with vaccines in general, with the MMR vaccine in particular, or with thimerosal or mercury.


Part of the fear that there is some environmental cause for autism comes from the steady increase in the incidence of diagnoses over the last 20 years. According to the CDC, in 2000 the prevalence of autism was 1 in 150, and in 2010 it had increased to 1 in 68. One reason we track the incidence (new cases per year) and prevalence (how many cases exist at one time) of diseases is as a warning system to alert us to a potential new risk. It is perfectly reasonable to ask questions about this increasing prevalence, which certainly suggests an environmental cause.

However, one reasonable question to ask is this – is the increase in the number of diagnoses being made due to an actual increase in the incidence of the disorder, or is it due to diagnostic definitions and behavior? I discussed this question previously at length. The short answer is that much of the increase in the number of diagnoses can be explained by a broadening of the definition, increased surveillance, and diagnostic substitution.

The data is not sufficient to rule out the conclusion that a subset of the increase is due to a real increase in incidence, but neither has a real increase been definitively demonstrated. It’s possible that it’s all an artifact of diagnosis.

Further, there is copious and increasing scientific evidence that autism is primarily a genetic disorder. Twin studies show a large genetic component. Autism is linked to many individual gene mutations, duplications, and deletions. Not surprisingly, the many specific genes linked to autism are involved in brain development – precisely the genes you would think would be involved in this kind of disorder. At this point there is no question that autism is predominantly a group of complex genetic disorders affecting the organization and connections of the neurons in the brain. This does not by itself rule out an environmental factor – environment can modify the expression and implications of specific genetic changes. The role of environmental factors, however, is certainly diminished by the apparent predominant role of genetics.

The final point to make about autism is that recent research has demonstrated that it can be diagnosed at younger and younger ages, if we look carefully. Several studies show that signs of autism can be detected as early as six months of age, and preliminary data suggests this may be possible as early as two months. Even more significant is a recent study suggesting that the brain changes found in autism must begin in early fetal development – in the womb.

To summarize, evidence suggests that the true incidence of autism is not increasing (although a small increase cannot be ruled out), that it is dominantly genetic (although an environmental factor cannot be ruled out), and that autism is already present in very young children and likely has its roots in early fetal development, before any vaccines are given. These lines of evidence all argue strongly against any association of autism with vaccines.

MMR and thimerosal

The MMR vaccine and thimerosal have been the key targets of the anti-vaccine movement, although their real focus is to blame vaccines in general. The MMR scare was triggered by the now-infamous Lancet study (since retracted) by the discredited researcher Andrew Wakefield. This story has been told multiple times here and elsewhere. To summarize, Wakefield’s small and preliminary study was found to also be likely fraudulent. Subsequent research has not found any link between the MMR vaccine and autism.

As the MMR story was crashing and burning, the antivaccine movement latched onto another boogeyman in the guise of thimerosal, a mercury-based preservative in some vaccines (although never in the MMR). Targeting thimerosal had the advantage of allying the antivaccine movement to the anti-mercury movement and some extreme environmentalists (most notably, RFK Jr.). The thimerosal scare was also given a boost by the book Evidence of Harm by David Kirby.

In the early 2000s, Kirby and other antivaccinationists argued that the increasing exposure to thimerosal in the vaccine schedule was the true cause of the apparent rise of autism rates. The last 14 years, however, have definitely disproven this “hypothesis.” Multiple studies have failed to show any correlation between thimerosal exposure, mercury exposure, or mercury excretion and the risk of developing autism. There is simply no link.

But the final nail in the coffin came as a result of the removal of thimerosal from the routine vaccine schedule around 2001. By 2002 children in the US were no longer receiving vaccines with added thimerosal as a preservative as part of the routine schedule. Some multi-dose flu vaccines still contain thimerosal, but these are optional, and most flu vaccines given in the US don’t contain thimerosal. Thimerosal is also still used in the manufacturing process of some vaccines, and so trace amounts may remain in the final product.

This, of course, gives deniability to dedicated antivaccine ideologues. But the undeniable truth is that exposure to thimerosal through vaccines was dramatically reduced around 2002 – to trace amounts for most children. At that time Kirby and others predicted that autism rates would plummet, confirming their claim that thimerosal was the cause of the apparent increase in rates. This is the one point on which we agreed – if thimerosal was a significant risk factor for the development of autism, and in fact was the cause of the increase in diagnosis rates, then those rates should plummet within 4-7 years following the removal of thimerosal from the vaccine schedule by 2002.

It has now been 12 years and autism rates have not plummeted. In fact, there has not been the slightest blip in the steady rise of diagnosis rates over the last decade. Removing thimerosal from the routine vaccine schedule has had zero detectable effect on the diagnosis of autism.

This is the absolute final nail in the coffin of the failed thimerosal/mercury hypothesis of autism. Only the motivated reasoning of the most dedicated antivaccine ideologue can deny this simple fact.


The recent meta-analysis is just icing on the cake of the body of scientific evidence that fails to demonstrate any link between vaccines in general, MMR, or thimerosal with autism or ASD. Scientific research also gives us many independent reasons to doubt such a link.

There is no scientific controversy here. As much as ideologues hate to hear this – the science is settled. This, of course, does not stop dedicated ideologues from fearmongering with false claims and pseudoscience in order to promote their agenda.

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 president and co-founder of the New England Skeptical Society, 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 contributes every Sunday to The Rogues Gallery, the official blog of the SGU.