A Comprehensive Review of the MMR Vaccine Studies: Many Years and Millions of People Reveal NO Link to Autism
Time to Move Forward: What Decades of Consistent Evidence Should Tell Us
Have you ever wondered what all those science articles about vaccines and autism really say? Look no further! A pediatrician, immunologist, epidemiologist, and public health scientist teamed up to break down all the articles into easy-to-understand summaries just for you.
We've been discussing this same question since the 1990s, and now we find ourselves revisiting it once again as the CDC, under new leadership, recently announced plans to conduct yet another study on vaccines and autism. This represents an unfortunate diversion of resources that could be better used studying the actual causes of autism or developing more effective supports for individuals with autism spectrum disorders.
Can we finally put this question to rest? We believe we can. Our goal with this article is to provide you with clear information that you'll feel confident sharing with your family, friends, and community members.
A Note on Autism Stigma: Before diving into the research, we must acknowledge that the persistent focus on vaccines as a potential cause of autism has unintentionally perpetuated harmful stigma around autism spectrum disorder. The fear-driven narrative that suggests we should risk deadly diseases to avoid autism implies that autism itself is something terrible to be feared—a damaging message that harms autistic individuals and their families.
Autism is a different way of experiencing and interacting with the world, bringing both challenges and unique strengths. By obsessively investigating vaccines as a potential cause while neglecting research into supports and accommodations, we've prioritized prevention over acceptance and understanding.
The autism community has repeatedly expressed concern about being used as a reason to avoid vaccination, especially when this has led to the resurgence of dangerous preventable diseases. As we review the evidence, let's remember that our goal should be both protecting children from preventable diseases AND supporting neurodiversity with dignity and respect.
A Technical Note: Autism is a commonly used shorthand term for autism spectrum disorder (ASD). While there is a technical distinction between the two, "ASD" is typically considered a medical term, whereas "autism"--the term that we use throughout this article– is more commonly used in everyday conversations.
Key Takeaways
The scientific consensus on vaccines and autism is clear and definitive: there is NO link between vaccines and autism. This conclusion is supported by:
1. Scale of Evidence: More than 20 major studies involving over 10 million children across multiple countries, populations, and decades have found that there is no link between vaccines and autism.
2. Diversity of Methods: Every type of scientific study design—from systematic reviews and meta-analyses to large cohort studies and case-control studies—has reached the same conclusion that there is no link between vaccines and autism.
3. Independent Replication: Research conducted by different scientific teams worldwide, using different methods and populations, has consistently found no link between vaccines and autism.
4. Response to Fraud: The original 1998 study suggesting a link between the MMR vaccine and autism was found to be fraudulent, its author lost his medical license, and the paper was retracted from the scientific literature.
TL;DR: There isn't a shred of scientific evidence that vaccines cause autism.
Parents can be confident that vaccinating their children protects them from serious diseases without increasing the risk of autism. The scientific evidence on this question is not merely suggestive—it is overwhelming and conclusive. Let’s discuss…
Study Designs at a Glance
Scientists use many different types of study designs to help understand potential causes of and treatments for diseases–each of which has its strengths and limitations. For example, case-control studies are useful for studying rare diseases, while cohort studies are often preferred when studying rare exposures. While no single study can definitively establish cause and effect, researchers have repeatedly used these different study designs–across different countries, age groups, time periods, etc.--to look at the relationship between the MMR vaccine and autism. No matter which study design they used, each study independently and consistently came to the same conclusion: the MMR vaccine does not cause autism.
Randomized Control Trial (RCT)
RCTs are a type of scientific experiment used to test the effectiveness or how well a treatment or intervention (like a vaccine) works. It is called randomized because the people who agree to participate in the study (aka participants) are randomly assigned to one of two groups in the study. The two groups of the study are the ‘treatment’ group or the participants who receive the treatment (like a vaccine) or intervention and the ‘control’ group or the participants who do not receive the treatment or intervention. The participants who partake in these types of studies understand all of the elements in advance and know that they are randomly put into one group or the other. The control group sometimes received a ‘placebo’ instead of the treatment or intervention. A placebo is a harmless replacement for the treatment being tested. As an example, if we pretend the study is about drinking apple juice for extra energy, the control group might be given a glass of water instead. Blinding is another term that may be part of RCTs. Blinding is when participants don’t know which group they are assigned to (treatment vs. control). It is called blinding because no one knows if they are receiving the treatment or the control. By randomly assigning participants and using blinding, RCTs aim to ensure that the results are reliable and not influenced by outside factors–sometimes called “bias”. This helps scientists determine whether a new treatment or intervention is truly effective.
People who spread vaccine disinformation often argue that new RCTs with placebo groups are needed to rule out side effects like autism. However, this is ethically impossible because it would require deliberately withholding a potentially life-saving vaccine from some participants. To illustrate, imagine proposing an RCT to test car seat safety—where one group secures their babies in properly tested car seats, while the other is told to hold their babies in their laps. For good reason, no ethical review board would ever approve such a study, and the same applies to vaccines.
Systematic Reviews and Meta-Analyses: A systematic review is a summary of existing research. It is a special type of literature review that uses strict criteria to search for relevant studies that can help answer a research question. These criteria are determined before the study begins to minimize the chance of bias when searching for and including studies, and must be clearly documented so the findings of the review can be reproduced. Systematic reviews summarize each study’s design, sample size, study population, and findings. A systematic review often uses a meta-analysis which is a study method that combines data from multiple studies on the same topic to find a more reliable overall result. It helps identify patterns, measure the strength of an effect, and provide a clearer picture of what the evidence shows. This method uses a standardized checklist to determine each study’s overall quality. so that poorer-quality studies receive less statistical weight than high-quality studies when developing a final answer to the research question of interest.
Cohort Study: A cohort study is a type of observational research study where scientists follow a group of people over time to see how different factors affect their health. The study is started by choosing a group with something in common, like age or a specific health condition. Over months or even years, a variety of relevant information is collected about the group's lifestyle, diet, and other habits that may help to better understand the outcomes of the study. By comparing the health outcomes of people with different behaviors or exposures, researchers can identify patterns and figure out what might increase or decrease the risk of certain diseases. This helps doctors, scientists and public health experts make better recommendations for keeping people healthy.
Case-Control Study: A case-control study is a type of observational research study used to find out what might cause a certain health condition. Researchers start by identifying two groups of people: one group with the condition (called "cases") and another group without the condition (called "controls"). The scientists then look back in time to see what factors might have been different between the two groups, such as lifestyle choices, environmental exposures, medical histories and sometimes even certain genes via genetic testing. By comparing these factors, scientists can identify potential causes or risk factors for the condition under evaluation. This type of study is especially useful for studying rare diseases or conditions that take a long time to develop.
Cross-Sectional Study: A cross-sectional study is a type of study that looks at a group of people at one point in time to see how common a condition, behavior, or trait is. It helps identify patterns and trends but can’t show cause and effect. These studies are often used in public health and medicine to understand risk factors, health issues, or opinions. Surveys and health check-ups are common examples.
Ecological Study: An ecological study is an observational study that looks at a large group of people from a specific population to understand health trends and outcomes. Researchers collect and analyze population-level data to help identify patterns and factors that affect the health of the population, such as lifestyle habits, environmental exposures, or genetic traits. These studies help doctors, scientists, and public health officials understand how common certain health conditions are and give ideas about what may be causing them. This type of study may lead to better prevention and treatment strategies for the whole community.
Case Series: A type of research that describes a group of patients with a similar condition or exposure but without a control group for comparison. Case series lack a control group, randomization, and blinding, making it impossible to determine whether an observed effect is due to the exposure or other factors. Since there’s no way to rule out coincidence or bias, case series can suggest associations but cannot establish cause and effect. These types of studies can generate ideas to study further but are frequently misused to suggest an association that isn’t real. The now-discredited, fraudulent study published in 1998 that kicked off the myth about autism and MMR vaccines was a case series.
Systematic Reviews and Meta-Analyses
Summary: This systematic review and meta-analysis included 338 studies to assess adverse events and safety of vaccines included on the CDC’s recommended immunization schedules in 2020, adding 189 new studies since 2014 when this review was last updated. Specifically, there was strong evidence that the MMR vaccine was not associated with an increased risk of autism. Additionally, among individuals who received a Tdap vaccination during pregnancy, there was no evidence of an increased risk of autism in their infants.
Summary: This systematic review included 138 different studies with over 23 million participants. It assessed MMR vaccine effectiveness and potential harms. The MMR vaccine showed high effectiveness in preventing measles, mumps, and rubella. Safety analyses of MMR vaccines found no link to autism, encephalitis, or other conditions.
Summary: This is a meta-analysis reviewing data from five cohort studies (1,256,407 children) and five case-control studies (9,920 children) that found no association between vaccination and the development of autism. Additionally, vaccine components such as thimerosal (mercury) and multiple vaccines, including MMR, showed no link to autism.
Summary: This systematic review of 12 studies comprehensively examined several potential links between the MMR vaccine and autism. The findings were clear: there was no difference in autism rates between children who received the MMR vaccine and those who did not. There was no evidence to support a causal relationship between rising MMR vaccination rates and autism prevalence. Four studies specifically investigated whether a specific autism variant was linked to the MMR vaccine and none found an association.
Cohort Studies
Summary: This was a nationwide cohort study of all Danish children (N=657,461) born between 1999-2010. Overall, kids who received the MMR vaccine were not at increased risk of an autism diagnosis compared to kids who did not receive the MMR vaccine. Even among kids who had several risk factors for autism, there was no evidence that the MMR vaccine triggered autism. With a total of 6,517 cases of autism detected over the study period, this is one of the largest studies exploring a link between the MMR vaccine and subsequent autism and would have statistical power to find even a small association between the MMR vaccine and autism if it existed.
Summary: This is a cohort study of 95,727 children who have older siblings to investigate the association between autism and receipt of the MMR vaccine. In this large sample, receiving the MMR vaccine was not associated with increased risk of autism, regardless of whether older siblings had autism. This means there was no evidence for an association between MMR vaccine and autism, even in children who may be at higher risk for autism.
On-time Vaccine Receipt in the First Year Does Not Adversely Affect Neuropsychological Outcomes (2010)
Summary: This is a cohort study of 1,047 US children aged 7-10 who were born between 1993-97. They underwent neuropsychological testing for 42 outcomes. Researchers looked at the children’s medical records to determine 1) their exposure to ethylmercury from thimerosal in vaccines they received during infancy, and 2) whether each vaccine was received within 30 days of the recommended age (on time) or a delayed schedule was used. On-time vaccination during infancy has no adverse effect on neuropsychological outcomes 7 to 10 years later.
Summary: This was a cohort study of 27,749 children attending 55 schools in Montreal, Canada to determine whether there is an association between pervasive developmental disorders and exposure to ethylmercury (thimerosal) or 1- or 2-dose MMR vaccinations. The findings of the study indicate no association between pervasive developmental disorders and high levels of thimerosal exposure or MMR vaccine dosage.
Summary: This cohort study analyzed MMR vaccination data of 535,544 children vaccinated between 1982 and 1986 in Finland to examine the association between MMR vaccination and neurological disorders. The results of this study found no evidence of increased risk of encephalitis or aseptic meningitis among vaccinated children. There was also no association between MMR vaccination, autism, and inflammatory bowel disease.
Summary: This was a nationwide cohort study of all children born in Denmark from 1991-1998 (537,303 children) comparing the risk of autism in children who had been vaccinated with the MMR vaccine compared to those who hadn’t. There was no association between receiving the MMR vaccine and autism. There was also no association between the age at the time of vaccination, the time since vaccination, or the date of vaccination and the development of autism.
Summary: This cohort study compared children (262 total) with pervasive developmental disorders who were pre- or post- MMR vaccination. Children were assessed with the standardized Autism Diagnostic Interview (ADI), allowing rigorous comparison of age at first parental concerns and rates of regression across samples. No evidence was found to support a distinct syndrome of MMR-induced autism or of “autistic enterocolitis.”
Summary: This was a cohort study that followed 1.8 million people in Finland who received the MMR vaccine between 1982 and 1996 to identify serious adverse events linked to vaccination. Using a nationwide surveillance system, researchers monitored for potential vaccine-related complications. Over the 14-year follow-up period, no cases of autism were associated with MMR vaccination.
Case-Control Studies
Early Exposure to the Combined Measles-Mumps-Rubella Vaccine and Thimerosal-Containing Vaccines and Risk of Autism Spectrum Disorder (2015)
Summary: This case-control study examined whether early exposure to the MMR vaccine and thimerosal-containing vaccinations influenced the risk of autism onset in a Japanese population. There were no significant differences in MMR vaccination between 189 autism cases and 224 matched controls. No convincing evidence was found in this study linking MMR vaccination to a higher risk of autism.
Summary: This case-control study compared 96 children with autism to 192 matched control children to determine whether there was a relationship between the MMR vaccine or single measles vaccine and autism. Children who received the MMR vaccine before their autism diagnosis had a lower risk of autism compared to those who were not vaccinated or those who received only the single measles vaccine. Overall, being vaccinated (regardless of vaccine type) was linked to a lower autism risk compared to not being vaccinated. The study provides evidence against the association of autism with either MMR or a single measles vaccine.
Summary: This case-control study examined whether measles vaccination was linked to autism by looking at signs of persistent measles infection or abnormal immune responses. The study analyzed 98 children with autism, comparing them to two control groups: 52 children with special educational needs but no autism and 90 typically developing children. No association between measles vaccination and autism was found.
Summary: This case-control study compared 54 children with autism and 34 control children to determine whether measles virus nucleic acids persist in children with autism compared to controls. There is no evidence of measles virus persistence in the peripheral blood mononuclear cells of children with autism spectrum disorder.
Summary: This is a matched case-control study in the UK. Cases (n=1,294) were born after 1973 and had a developmental disorder diagnosed between 1987-2001. They were matched to controls (n=4,469) who were the same age and sex and sought care at the same general practice clinic. Then, researchers recorded any MMR vaccines received before the developmental disorder diagnosis in cases, or at the comparable age for the matched controls. They found no association between MMR vaccination and developmental disorders, including autism.
Summary: This case-control study compared the ages at first MMR vaccination between 624 children with autism and 1,824 matched controls without autism. The age at the time of first MMR administration was similar among case and control children.
Ecological Studies
Summary: This ecological study analyzed data from a general practice research database in the UK (1988–1993) to assess changes in autism risk and its potential link to MMR vaccination. The findings showed no correlation between the prevalence of MMR vaccination and the rapid increase in autism cases over time, providing further evidence against a causal relationship.
Summary: This ecological study examined data from children born between 1980 and 1994 who were enrolled in Kindergarten in California to investigate whether MMR vaccination rates were linked to rising autism cases. The analysis found no evidence of a correlation between MMR immunization coverage in young children and the increase in autism diagnoses over time.
Autism and measles, mumps, and rubella vaccine: no epidemiological evidence for a causal association (1999)
Summary: This ecological study examined whether the MMR vaccine was linked to autism by analyzing trends in incidence and age at diagnosis following its introduction in the UK in 1988. Researchers looked for shifts in autism rates that could suggest a causal relationship. The analysis found no evidence to support a link between MMR vaccination and autism.
Other Vaccines and Autism
Maternal Influenza A(H1N1) Immunization During Pregnancy and Risk for Autism Spectrum Disorder in Offspring: A Cohort Study (2020)
Summary: This cohort study used nationwide registers in Sweden to evaluate the risk of autism in children of mothers who were vaccinated against H1N1 (“swine flu”) during pregnancy. Prenatal exposure to H1N1 immunization in pregnant mothers was not associated with an increased risk for autism in their children.
Summary: This is a cohort study obtained maternal Tdap vaccination, clinical autism diagnoses, and other pregnancy-related information from electronic medical records to examine the association between prenatal tetanus, diphtheria, acellular pertussis (Tdap) vaccination and the risk of autism in mothers’ offspring. The findings of this study found no association between maternal vaccination and increased autism risk in their children.
Summary: This was a cohort study of 196,929 children born at a Kaiser Permanente Northern California hospital between January 1999 and December 2010. The researchers obtained data on maternal influenza infection and influenza vaccination during pregnancy from patient databases, as well as clinical diagnoses of autism from electronic medical records. They found no association between maternal influenza infection or vaccination and increased autism risk in their children.
Okay, so why are autism rates rising?
Many people point to increasing autism rates as evidence that something new must be causing autism. However, scientists have identified several factors that help explain this increase, independent of environmental changes such as vaccination:
Expanded Diagnostic Criteria: The clinical definition of autism has broadened significantly over time, now encompassing a wider spectrum of behaviors and symptoms than in previous decades.
Improved Awareness and Recognition: Healthcare providers, educators, and parents are much more knowledgeable about the signs of autism today than they were in the past, leading to more frequent identification, particularly among historically underrepresented racial and ethnic groups.
Earlier Diagnosis: Children are being diagnosed at younger ages, increasing the total number of identified cases at any given time.
Diagnostic Substitution: Many children who would have previously received other diagnoses (such as intellectual disability) are now being correctly diagnosed with autism.
Required Reporting: More comprehensive tracking systems and active surveillance have improved our ability to count cases.
A recent cross-sectional “study” suggested an association between the number of clinical visits for vaccinations and increased risk of neurodevelopmental disorders such as autism. However, this study had many problems with its methods and analysis (so many problems, in fact, that we fully unpacked it here). Among the issues, the authors failed to account for these temporal trends in autism screening, diagnosis, and reporting – all of which help explain their biased estimates of autism prevalence. Scientifically rigorous, peer-reviewed studies published in reputable medical journals that have carefully controlled for these factors find that the true prevalence of autism has remained relatively stable, increasing only slightly over time.
Key Takeaways
The myth that vaccines cause autism gained mainstream attention from a now-discredited study published in 1998. This study was found to be fraudulent, the lead physician (aka Voldemort or “he who shall not be named”) of the study lost his medical license, and the paper was retracted. Despite this, the myth persists, fueled by misleading information on social media. In response to the fraudulent study, the scientific community has repeatedly confirmed that vaccines are safe and do not cause autism.
Over the years, extensive research has consistently shown that vaccines do not cause autism. This conclusion is supported by numerous large-scale studies. One of the most significant studies, published in 2013, examined the number of antigens children received in their first two years of life and found no difference between children with autism and those without. Additionally, the ingredient thimerosal, a mercury-based preservative once used in vaccines, has been thoroughly investigated and found to have no link to autism.
Parents can rest assured that vaccinating their children is a crucial step in protecting them from serious diseases without increasing the risk of autism.
Stay Curious,
Unbiased Science
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Want to learn even more? Check out these free review articles for additional reading:
The MMR Vaccine and Autism (DeStefano F & Shimabukuro TT; 2019)
Vaccines and Autism: A Tale of Shifting Hypotheses (Gerber JS & Offit PA; 2009)
The Problem with Dr. Bob’s Alternative Vaccine Schedule (Offit PA & Moser CA; 2009)
Immunization Safety Review: Vaccines and Autism (Institute of Medicine; 2004)
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I teach this with a case study from NSTA; one comment from a student (with autism) she wondered why don’t mention that Autism isn’t a death sentence, but measles can be.
Thank you, thank you!! When a parent says they’ve done their research , your compilation should be the minimum reading requirement. Sadly, vaccines can be the easy scapegoat to blame when a child has a divergent social and language development because general development up to that point ( age 1-3) can seem quite typical otherwise . The problem was there all along and there are many genetic explanations now.
I hope parents can feel reassured by this and choose to protect their children from serious vaccine -preventable illness complications.