Variations on a theme: The evidence refuting the false claim that vaccines cause autism
One of the most enduring narratives about vaccine safety is the alleged link between childhood vaccination and autism spectrum disorder (ASD). A study published in 2020 showed that more than one in four parents of children with ASD believed it was caused by vaccines.
This belief directly impacts public health, with data showing that the fear of vaccines causing autism in children is an important driver of vaccine hesitancy. Indeed, children who are diagnosed with ASD go on to receive fewer vaccines than children without such a diagnosis. Furthermore, the younger siblings of children with autism tended to receive fewer vaccines than the younger siblings of children without ASD. This suggests that, when an older child is diagnosed with ASD, parents become reluctant to vaccinate the rest of their children.
Therefore, determining whether there is any scientific basis to the belief that vaccines may cause autism is of paramount importance for concerned parents and their children. Fortunately, a large body of evidence is available. We review what is known in this article.
Contents of this article
- How did the narrative linking vaccines and autism begin?
- Childhood vaccination and autism: what the data say
- Multiple vaccinations and autism
- Thimerosal and autism
- Recurrent inaccurate claims about vaccination and autism
How did the narrative linking vaccines and autism begin?
As Health Feedback explained in a previous Insight article, it was the publication of a now-retracted study by Andrew Wakefield in 1998 laid the foundation for this narrative. In this study, Wakefield studied twelve children with pervasive developmental disorder (PDD)—a form of ASD—and claimed to have discovered a link between their PDD and vaccination with the measles, mumps, and rubella vaccine (MMR).
The paper benefited from extensive media coverage at the time and sparked significant interest in the research community. Subsequent investigation by reporter Brian Deer revealed that the study by Wakefield et al. was marred by data manipulation, undisclosed conflicts of interest, and ethical breaches.
This, together with research by other groups failing to reproduce Wakefield’s result, led the British General Medical Council (GMC) to launch an investigation. The GMC found Wakefield guilty of “serious professional misconduct” and struck him off the medical register in 2010. The same year, the journal that published the study, The Lancet, retracted the study.
But claims of a causal association between childhood vaccinations and ASD keep surfacing (more on that later). However, all reliable clinical evidence shows that getting your children vaccinated doesn’t increase the risk of developing any form of autism.
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Childhood vaccination and autism: what the data say
The scientific community focused much of its research effort on the MMR vaccine since the study by Wakefield et al. had pointed the finger at the MMR vaccine as a culprit for autism. Data accumulated over the past twenty years unambiguously show that the MMR vaccine doesn’t cause autism, as Health Feedback explained previously.
A meta-analysis showed MMR vaccination didn’t increase the risk of developing ASD. A study on more than 500,000 children in Denmark also showed that the MMR didn’t cause an increase in the rate of autism.
Another study monitored the incidence of ASD among children in Canada from 1987 to 1998. This time period is interesting because it is a period where the children’s exposure to MMR increased. This is because in Canada, children received a single dose of MMR vaccine until 1996, when an update in the childhood vaccination schedule increased the number of doses to two. If the MMR vaccine had caused autism, one could hypothesize that a higher dose would increase the number of ASD cases. However, the researchers found that the increase in MMR vaccine dose didn’t change the incidence of ASD .
Numerous other studies likewise showed that the MMR vaccine doesn’t put children at an increased risk of developing ASD[4,6-10].
One limitation of these studies is that most of them only compared children who got the MMR vaccine to children who didn’t, regardless of their vaccination status for other childhood vaccines. In other words, the children from the group who didn’t receive the MMR vaccine may well have received other vaccines. Thus, the majority of these studies don’t include a fully unvaccinated control group. To reliably answer the question of whether any vaccine is a risk factor for autism, a fully unvaccinated group would be important.
This question was addressed in 2012 by, Hviid et al., who published a study comparing the risk of autism between children who didn’t receive any childhood vaccines and those who only received the MMR vaccine. The number of children is small (a few dozen), which may limit the strength of the conclusion, but the authors found that the MMR vaccine didn’t increase the risk of autism compared to children fully unvaccinated.
Another limitation is that most of the studies only focused on the MMR vaccine. Thus, data on other childhood vaccines are limited. However, Uno et al. assessed the risk of autism associated with several childhood vaccines in a case-control study of 354 children diagnosed with ASD between 1984 and 1992. Having assessed the individual vaccines for measles, mumps, rubella, polio, B-encephalitis and tuberculosis, as well as the combination MMR and diphtheria–pertussis–tetanus vaccines, the researchers found that none of them increased the risk of autism.
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Multiple vaccinations and autism
Children typically receive several vaccines as part of their routine immunization. The 2024 childhood vaccination schedule by the U.S. Centers for Disease Control and Prevention (CDC) recommends twelve vaccines before the age of fifteen months. A study from 2011 reported that the top concerns of U.S. parents related to vaccination included their children getting too many vaccines during childhood and the fear of a link between vaccines and autism. To answer parents’ concerns, it is important to establish what the scientific data say on childhood multiple vaccinations and ASD.
Smith et al. compared children who received at least 10 vaccines within the first seven months of life and children who received less than six vaccines during that period. While the authors didn’t look at autism diagnosis, they found that the children within the most vaccinated group performed similarly or better in several neuropsychological tests assessing speech and languages, verbal memory, behavior regulation, tics, among others.
Uno et al. compared the childhood vaccination history of children diagnosed or not for ASD and found that a higher number of vaccine doses received during the first months of life didn’t increase the risk of having ASD.
However, simply counting the number of doses received by children isn’t enough. This is because the doses from different vaccines contain different amounts of antigens. Antigens are molecules that the immune system is able to recognize and target and are the active pharmaceutical ingredients of vaccines. It’s because vaccines contain antigens of specific pathogens that they are able to train the immune system into detecting and neutralizing this pathogen later on.
So, receiving a dose of vaccine with only one type of antigen may not be equivalent, for the immune system, to receiving a vaccine dose with multiple antigens. Therefore, it is important to assess the effect of the number of antigens instead of the number of doses on children’s neuropsychological development.
DeStefano et al. provides estimates of the number of antigens administered to children by type of vaccine. For example, one dose of MMR has 24 different antigens, whereas one dose of varicella vaccine has 69 different antigens. The authors found that the amount of vaccine antigen received by a child during the first two years of life didn’t change the risk of developing ASD .
Another study also found that increasing the amount of vaccine antigen received by children didn’t affect other neuropsychological outcomes, such as general intellectual function, speech and language, verbal memory, attention and executive function, tics, achievement, visual-spatial ability, and behavior regulation.
In line with the scientific evidence presented above, health agencies like the CDC and medical organizations such as the American Academy of Pediatrics and the American Academy of Child and Adolescent Psychiatry state that there is no link between vaccination and autism and recommend childhood vaccination.
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Thimerosal and autism
We showed in the previous sections that vaccines don’t cause autism. Nevertheless, the claim persisted, with some pointing the finger this time at thimerosal, an ingredient in some childhood vaccines, as a cause of autism. Thimerosal has been used in biological products for decades as a preservative to prevent the contamination of the product with harmful microorganisms. Deadly cases of contamination of multi-dose vaccine vials in the past illustrate the necessity to add preservatives to vaccines.
First and foremost, it is important to emphasize that no childhood vaccines have used thimerosal for the past twenty years in the U.S. The CDC explains that manufacturers have withdrawn thimerosal from vaccine composition since 2001 due to public concern. Furthermore, common childhood vaccines such as the MMR or chickenpox never included thimerosal in their composition. Thus, current fears over the effect of thimerosal on children’s cognitive development are moot.
To understand what may have cast doubt on the safety of thimerosal, we need to provide some background information on the chemistry of mercury.
Mercury appears in its pure form, called elemental mercury, or bound to other molecules. The chemical properties and the safety profile of mercury for human health may vary depending on the type of bound molecules.
One form of mercury is called methylmercury, because the mercury atom is bound to a methyl group, made up of one atom of carbon attached to three hydrogen atoms. According to the U.S. Environmental Protection Agency, methylmercury is the form that most people encounter in the U.S., mainly through eating fish and shellfish containing the compound. Methylmercury is highly toxic. Its half-life in the blood is 45 to 60 days and the compound may remain trapped in the central nervous system. Prenatal exposure to amounts of methylmercury above the safety level increases the risk of severe neurological disabilities.
One famous example of the devastating health effects of methylmercury is the Minamata disaster in Japan, whose inhabitants were exposed to toxic levels of methylmercury in 1956. An investigation revealed that a local factory had been releasing mercury into the bay of Minamata.
Once ingested by living organisms, the mercury was converted into methylmercury, and through the food chain, accumulated in fish that were then consumed by the local population. People poisoned with methylmercury developed numerous neurological disorders, such as sensory disturbance, trouble with balance, and impaired movement, among other symptoms.
The harmful effects of methylmercury poisoning on the nervous system might be the source of some people’s concerns about the presence of mercury in vaccines.
However, thimerosal doesn’t contain methylmercury, but a different form of mercury called ethylmercury, where the atom of mercury is bound to an ethyl group (comprising two atoms of carbon and five of hydrogen). The half-life of ethylmercury in the blood is much shorter than the half-life of methylmercury: three to seven days instead of forty-five to sixty days. Thus, the ethylmercury is rapidly excreted from the body, doesn’t accumulate, and is much less likely to cause health issues. According to the World Health Organization (WHO), there is no evidence that the amount of thimerosal used in vaccines poses a health risk.
Furthermore, an ample body of scientific results show that exposure to thimerosal isn’t associated with autism. A study measured the prevalence of autism among Canadian children born and vaccinated between 1987 and 19985. In this period of time, the presence of thimerosal in childhood vaccines progressively declined and it was completely removed in 1996.
If thimerosal were a risk factor for autism, we would expect to see a decline in autism as the exposure to thimerosal decreased. However, this isn’t what happened. The prevalence of autism was in fact lower when exposure to thimerosal was higher. Thus, this observation strongly suggests that thimerosal is not a risk factor for ASD. In addition, other studies comparing the risk of autism among children exposed to thimerosal or not confirmed that this compound doesn’t cause ASD or other neurological developmental disorders[15-22].
In summary, thimerosal doesn’t cause autism. Contrary to methylmercury, the ethylmercury found in thimerosal is rapidly excreted from the body and doesn’t accumulate in the central nervous system. Furthermore, it has been twenty years since thimerosal was used in childhood vaccines in the U.S.
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Recurrent inaccurate claims about vaccination and autism
In spite of all the scientific evidence presented above that vaccines don’t cause autism, claims that it does repeatedly appear on social media. We will summarize the most common claims and explain why they are inaccurate in this section.
Popular claim #1:
Unvaccinated children have lower rates of autism than vaccinated children
A recurring claim is that the prevalence of autism is lower among the unvaccinated population. As we explained earlier, most of the studies investigating the alleged link between vaccines and autism didn’t include a fully unvaccinated group This allowed some to argue that it was possible children who received no vaccines at all were less likely to develop autism than children who received any vaccine, and that such studies didn’t allow researchers to identify such a difference
But as we mentioned earlier, at least one published study aimed at addressing this weakness did include a fully unvaccinated group of children, and it found that their chances of developing autism weren’t different from that of the vaccinated children.
Furthermore, this claim isn’t supported by scientific evidence. Those who spread this claim have cited various scientific studies as supporting evidence. We analyzed some of these studies in detail in previous reviews.
Briefly, these studies targeted populations such as homeschooled children that aren’t representative of the general pediatric population, as illustrated by a much higher percentage of unvaccinated children compared to the general U.S. population. In addition, these studies didn’t correct for important confounders, such as potential differences in health-seeking behavior between parents of unvaccinated children and those of vaccinated children.
Controlling for differences between vaccinated and unvaccinated children is important, as vaccination status itself is associated with other factors that can influence health outcomes but don’t result from vaccination itself. For example, vaccinated children are more likely to see a doctor when unwell compared to unvaccinated children for various reasons, such as socioeconomic status and ability to access healthcare services[23,24].
As a result, vaccinated children are much more likely to be diagnosed with medical conditions, but this doesn’t necessarily mean that they are more likely to develop such conditions in the first place.
An alternative version of the claim is that Amish communities are unvaccinated and don’t develop autism. Again, we went over this claim in the past and found that it is inaccurate. First, Amish communities do vaccinate, albeit to a lesser extent than the general population. Thus, Amish communities cannot be seen as a fully unvaccinated “control population”. Second, records show that autism also occurs among Amish children.
Popular claim #2:
Vaccines contain mercury and aluminum that are toxic and cause autism
The claim that mercury in vaccines causes autism is recurrent. As we explained in detail above, childhood vaccines don’t contain mercury. Furthermore, the claim that mercury in vaccines causes autism stems from a misconception about the properties of different forms of mercury. The mercury used in the preservative thimerosal, formerly present in some U.S. childhood vaccines, is ethylmercury, which is rapidly excreted from the body and doesn’t increase the risk of developing autism, as numerous studies cited above demonstrate.
Alternate versions of the claim focus on another metal, aluminum. Some vaccines use aluminum salts as adjuvants, which are substances that boost the immune response to the vaccine. As we explained in a previous review, these aluminum salts are too big to cross from the blood to the brain. Furthermore, the amount of aluminum in vaccines is much lower than what toddlers ingest through diet, hence there is no reason to believe that the amount in childhood vaccines is harmful.
Popular claim #3:
The rise in the number of childhood vaccines is associated with a rise in autism compared to the past
This claim relies on the observation that autism diagnoses in the U.S. have increased over recent decades, while the number of vaccines in the U.S. childhood vaccination schedule also increased (see here and here), implying that the more vaccines children get, the more likely they are to develop autism. We reviewed this claim in previous reviews.
Some proponents of that claim explained this by the alleged mechanism that too many vaccines too soon would overwhelm the immune system, affect the nervous system and cause autism.
It is following this rationale that pediatrician Paul Thomas, who had his license suspended by the Oregon Medical Board for having failed to properly vaccinate children, came up with his own vaccination schedule that spaces out vaccinations.
Thomas’ vaccination schedule strongly implies that multiple vaccinations at an early age is a cause of autism. Indeed, he recommends parents who have cases of ASD in their family to “delay vaccines until at least age five” and claims that children visiting his practice have a “significantly lower rate of autism”
However, this reasoning is flawed. The fact that two events occur simultaneously doesn’t mean that one caused the other. As we explained in another Insight article, correlation alone is insufficient for establishing causality.
In fact, we provided evidence in this review that multiple vaccinations don’t cause autism. As we explain above, data show that children who were exposed to a higher amount of vaccine antigen during childhood aren’t at a higher risk of developing ASD than other children.
Furthermore, multiple vaccinations don’t overwhelm the immune system. The children’s immune system is able to cope with the amount of antigens present in vaccines. In fact, they encounter much more antigens through exposure to pathogens in their everyday life.
While the incidence of ASD has increased in the last decades, scientific evidence doesn’t suggest that this increase is related to the increase in the number of childhood vaccinations. Rather, the increase in the incidence of ASD was mostly due to changes in diagnostic criteria, as we explained in a previous review. The main reason for this is that the diagnostic criteria for autism broadened significantly over the years. This means that more children are now diagnosed with ASD when they wouldn’t have qualified under the earlier criteria.
Moreover, the data show that the increase in autism diagnoses was accompanied by a decrease in diagnoses of “intellectual disability”. This suggests that people who would have been categorized as intellectually disabled in the past are now more accurately diagnosed as autistic, thanks to our improved understanding of neurodevelopmental disorders.
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Although opponents of vaccination blame childhood vaccinations for an increase in autism and other neurodevelopmental disorders, the scientific results available to date overwhelmingly show that vaccines don’t cause autism. The narrative linking vaccines with autism relies on flawed scientific studies, some of which had to be retracted owing to data manipulation or insufficient results. In fact, we showed that studies comparing children who received a given vaccine and others who didn’t reported no elevated risk of autism after vaccination.
A misunderstanding of chemistry also served to promote the claim that some vaccine ingredients like thimerosal could cause autism. However, studies have found no association between thimerosal and autism, and it has since been removed as a vaccine ingredient in most vaccines, at least in the U.S. and Europe.
While there has indeed been a rise in the number of autism diagnoses in the last decade, the evidence points not to vaccines as the cause, but to a broader diagnostic criteria of autism and increased awareness of the condition now as compared to the past.
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