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Vaccine FAQ

Why Have Vaccines Been Ruled Out as a Cause of Autism?

Updated 1 February 2024


Autism, which is also known as autism spectrum disorder or ASD, is a neurological condition, meaning that it relates to how a person’s brain works. This means a person with autism processes information differently than a person without autism, also called neurotypical. The ways in which autism presents vary, but people with autism are diagnosed due to difficulties with typical social skills, a preference for repetition and routine, and communication disabilities (whether they are speaking or non-speaking). While some people with autism have obvious developmental and intellectual disabilities, others do not and may not get an autism diagnosis until later in life, if at all.[1] It is important to know that autism is not an illness or a disease, and therefore autistic people need understanding and support, not cures or treatments. 

According to the Centers for Disease Control and Prevention (CDC), autism prevalence (the number of existing cases) in the United States has increased in the last 20 years. Prevalence rose from 1 in 150 children in 2000 (children born in 1992) to 1 in 36 in 2020 (children born in 2012).[2] Over the decades, bad science has fueled conspiracy theories that blame vaccines as the reason for this increase. However, the scientific community has shown that the increase in prevalence is due to several factors: better diagnostic tools for detecting autism in children, better access to care that leads to more children being diagnosed, and more societal acceptance of autism (meaning parents are more likely to share their children's diagnoses). 

While genetic and environmental reasons for autism have been found, vaccines are not among those reasons.[3] In fact, one vaccine, the rubella vaccine has been found to have a protective effect in preventing rubella during pregnancy. Rubella during pregnancy can lead to children born with congenital rubella syndrome (CRS), which include symptoms of autistic behavior and developmental delay.[4] The rubella vaccine is part of the combined measles, mumps, rubella (MMR) vaccine. 

A Matter of Timing 

Perhaps the easiest answer to why vaccines were associated with autism to begin with is the age at which childhood vaccines are given and when the first signs of developmental delay are typically seen. In the United States, the vaccine schedule recommends certain vaccines be given between birth and twelve months of age. At the same time, signs of a developmental delay – like the delays seen with autism – show up around the time a child should start to speak and interact more fully with people around them. 

This is a classic case of confusing causation with correlation. People will look at that timing and associate one thing (vaccines) with another (an ASD diagnosis). This is also known as the Post Hoc Ergo Propter Hoc fallacy, which can be summed up this way: "Since event Y followed event X, event Y must have been caused by event X...” even when the two events are completely unrelated. In this case, X and Y are said to be correlated. To find out if the correlation is just that, a correlation, legitimate and well-conducted scientific studies are needed. 

The timing of the estimates of autism prevalence are also important. Before the 1990s, prevalence was estimated using limited studies. And, as stated above, the behavioral characteristics used to diagnose autism have changed. As a result, it is not scientifically proper to compare current estimates based on modern science with estimates done at a time when autism was not well understood, or when society did not count or recognize autistic individuals. 

The Wakefield Study on MMR 

While vaccines have been blamed for many bad outcomes throughout history, it wasn’t until an article in The Lancet that autism was fully introduced as a possible side effect of vaccination. The article was written by Andrew Jeremy Wakefield and others at the Royal Free Hospital and Medical School in London. One of the major problems with the Wakefield et al. article was that it did not have a control or comparison group of unvaccinated children, and only looked at a very small number of children in the total study.  

Furthermore, investigative journalist Brian Deer discovered that patient records were altered, that Mr. Wakefield was paid to find something wrong with the MMR vaccine, and that Mr. Wakefield was looking to patent his own measles vaccine.[5,6] The article by Mr. Deer exposes how Andrew Wakefield falsely linked the MMR vaccine to autism, starting a global health scare. 

Published in The Lancet in 1998 and retracted in 2010, Wakefield's paper reported on 12 children with developmental challenges, claiming a connection between the MMR vaccine and behavioral symptoms. However, an in-depth investigation by Deer uncovered many inconsistencies and ethical breaches in Wakefield's research. For example, the onset of behavioral symptoms in some children occurred before the MMR vaccination, contrary to what was reported. 

According to Mr. Deer, Wakefield was driven by undisclosed financial incentives, and was found to have made-up data to support a lawsuit against vaccine manufacturers. He had been paid to find a link between the vaccine and a new syndrome combining brain and bowel diseases. Review of the original medical records of the children in the study contradicted many of Wakefield's claims, including the timelines of symptom onset and the nature of the symptoms themselves. 

The investigation revealed that most children in the study did not fit the profile Wakefield claimed. Some had pre-existing developmental issues, while others showed symptoms unrelated to the vaccine. The misrepresentation of these children's cases, along with Wakefield's undisclosed financial conflict of interest, led to his and some of his co-authors' removal from the medical register. This means they cannot legally practice medicine in the United Kingdom. 

[The full article by Brian Deer, How the case against the MMR vaccine was fixed, can be read here:

This scandal discredited Wakefield, but the damage to public confidence in the MMR vaccine was done. The article ignited a worldwide vaccine scare, resulting in declining vaccination rates and outbreaks of preventable diseases. Despite being professionally discredited, Wakefield still has a following among some who oppose vaccinations. The Wakefield vaccine fraud serves as a cautionary tale about the importance of ethical conduct and thorough peer review in medical research. 

Later epidemiological investigations that were large and included control groups, and other stricter methods of assessing cause and effect, have not shown a link between the MMR vaccine – or any vaccine in the childhood schedule – and autism.[7,8,9] 

Turning to Thimerosal 

Thimerosal is a substance added to vaccines in small, non-toxic amounts to reduce the risk of bacterial contamination. Thimerosal contains a different form of mercury called ethylmercury. This is important to note because ethylmercury is processed by the human body and made harmless so it can be naturally and easily eliminated from the body. Thimerosal does not have the more dangerous form of mercury, methylmercury. This is the mercury-containing chemical associated with toxicity from eating contaminated fish or drinking contaminated water. (For comparison, think of the difference between ethanol, which is found in beer and wine, and methanol, which can cause blindness if a person ingests as little as two teaspoons.) 

Thimerosal has been used since the 1930s to prevent bacterial growth in medical solutions, so we have almost 100 years of information on its toxicity.[10] Nevertheless, thimerosal became the next target of anti-vaccine groups looking to link vaccines to autism in the late 1990s and early 2000s. They raised concerns about the potential neurotoxic effects of mercury exposure from thimerosal-containing vaccines (TCVs) and a possible link to autism spectrum disorders (ASD).  

Extensive research, including epidemiological studies and reviews, has been conducted to investigate this potential association. The current scientific consensus, supported by multiple studies, shows there is no evidence of a causal relationship between thimerosal-containing vaccines and autism. Studies have examined populations with varying levels of exposure to thimerosal and have found no significant association between TCVs and autism.[11,12,13,14] For instance, a study conducted in Poland found no significant association between TCV exposure and autism after adjusting for potential confounders. Another large-scale study in Denmark compared children vaccinated with thimerosal-containing vaccines to those vaccinated with thimerosal-free vaccines, and found no difference in the risk of autism spectrum disorders. Additionally, animal studies, such as those involving infant rhesus macaques (a type of monkey), have not seen autism-like behavior or neuropathology following administration of thimerosal-containing vaccines. 

Despite the scientific evidence refuting the link between thimerosal and autism, the controversy has had a lasting impact on public feeling and vaccine hesitancy. In response to public concern, thimerosal was removed from the majority childhood vaccines in the United States in 2001.[15] (Childhood vaccines in multi-dose vials, like some influenza vaccines, still have thimerosal.) However, autism prevalence did not drop since 2001, and many studies since have continued to show no association between thimerosal-containing vaccines and autism.[16] 

If Not Vaccines, Then What? 

While the prevalence of autism has been increasing, the same science that looked at an association between vaccines and autism tells us we are probably measuring the prevalence of autism closer to what it has always been. This means that neurodiverse people, including those with autism, have always been here, and we are only now starting to understand how large that population is. While there is some evidence of a multifactorial process involving a complex interplay of genetic and environmental factors can lead to symptoms of autism in a small proportion of cases. For most, science is moving toward agreement that autism is simply a natural variation of how brains develop. Further research is needed to fully understand these factors and their interactions, which could lead to more effective ways to diagnose ASD and provide accommodation, services, and care for those affected by it.[17] 


Autism is a complex disorder that presents itself in many ways in different people. Some people will be affected greatly, needing constant care and support services throughout their life. Others will be affected minimally, with only a few accommodations needed. In any case, it is important to emphasize that people with autism still have feelings, hopes, and dreams, just like anyone else. It’s just that their interface with the world – their brain – functions differently than most people. The resources spent on chasing a cause (e.g. vaccines[18]) already disproved could be better spent on services for autistic individuals and their caretakers.  


Autism Self Advocacy Network:  

Center for Autism Spectrum Disorders at Children’s National Hospital:  

Autism Integrated Care Program at Childrens Hospital of Philadelphia:  


1. National Health Service. What is autism? [Internet]. London: National Health Service; [updated 2022 Sep 7; cited 2024 Jan 29]. Available from:  

2. Centers for Disease Control and Prevention. Data & Statistics on Autism Spectrum Disorder. [Internet]. Atlanta, GA: Centers for Disease Control and Prevention; 2023 Apr 4 [cited 2024 Jan 29]. Available from:

3. Nordahl CW, Lange N, Li DD, et al. Brain enlargement is associated with regression in preschool-age boys with autism spectrum disorders. Proc Natl Acad Sci U S A. 2011;108(50):20195-20200. doi:10.1073/pnas.1107560108 

4. Hutton J. Does Rubella Cause Autism: A 2015 Reappraisal?. Front Hum Neurosci. 2016;10:25. Published 2016 Feb 1. doi:10.3389/fnhum.2016.00025 

5. Godlee F, Smith J, Marcovitch H. Wakefield’s article linking MMR vaccine and autism was fraudulent BMJ 2011; 342 :c7452 doi:10.1136/bmj.c7452 

6. Rao TS, Andrade C. The MMR vaccine and autism: Sensation, refutation, retraction, and fraud. Indian J Psychiatry. 2011;53(2):95-96. doi:10.4103/0019-5545.82529 

7. Smeeth L, Cook C, Fombonne E, et al. MMR vaccination and pervasive developmental disorders: a case-control study. Lancet. 2004;364(9438):963-969. doi:10.1016/S0140-6736(04)17020-7 

8. Madsen KM, Hviid A, Vestergaard M, et al. A population-based study of measles, mumps, and rubella vaccination and autism. N Engl J Med. 2002;347(19):1477-1482. doi:10.1056/NEJMoa021134 

9. Taylor B, Miller E, Lingam R, Andrews N, Simmons A, Stowe J et al. Measles, mumps, and rubella vaccination and bowel problems or developmental regression in children with autism: population study BMJ 2002; 324:393 doi:10.1136/bmj.324.7334.393 

10. U.S. Food and Drug Administration. Thimerosal and Vaccines. [Internet]. Silver Spring, MD: U.S. Food and Drug Administration; [updated 2023; cited 2024 Jan 29]. Available from:  

11. García-Fernández L, Hernández AV, Suárez Moreno V, Fiestas F. La evidencia acerca de la controversia de las vacunas que contienen timerosal y su asociación con el autismo [Addressing the controversy regarding the association between thimerosal-containing vaccines and autism]. Rev Peru Med Exp Salud Publica. 2013;30(2):268-274. 

12. Mrozek-Budzyn D, Majewska R, Kiełtyka A, Augustyniak M. Brak zwiazku miedzy ekspozycja na szczepionki zawierajace tiomersal i rozwojem autyzmu u dzieci [Lack of association between thimerosal-containing vaccines and autism]. Przegl Epidemiol. 2011;65(3):491-495. 

13. Hviid A, Stellfeld M, Wohlfahrt J, Melbye M. Association between thimerosal-containing vaccine and autism. JAMA. 2003;290(13):1763-1766. doi:10.1001/jama.290.13.1763 

14. Stehr-Green P, Tull P, Stellfeld M, Mortenson PB, Simpson D. Autism and thimerosal-containing vaccines: lack of consistent evidence for an association. Am J Prev Med. 2003;25(2):101-106. doi:10.1016/s0749-3797(03)00113-2 

15. Children's Hospital of Philadelphia. Vaccine Ingredients – Thimerosal. [Internet]. Philadelphia, PA: Children's Hospital of Philadelphia; [updated 2020 Jun 1; cited 2024 Jan 29]. Available from:

16. Institute of Medicine (US) Immunization Safety Review Committee. Immunization Safety Review: Vaccines and Autism. Washington (DC): National Academies Press (US); 2004. 

17. Amaral DG. Examining the Causes of Autism. Cerebrum. 2017;2017:cer-01-17. Published 2017 Jan 1. 

18. Knopf A. Time to remember: Vaccines don't cause autism. The Brown University Child and Adolescent Behavior Letter. 2021;37(7):9-10. doi:10.1002/cbl.30559