Do vaccines cause autism?

Vaccines do not cause autism. It is possible that the timing of an autism diagnosis or onset of autism symptoms might coincide with the recommended vaccine schedule for children, but this is a coincidence, not a cause.

Vaccines and autism 

Scientists have conducted extensive research all over the world over the last two decades to examine the link between childhood vaccinations and autism. The result of this research is clear: vaccines do not cause autism. 

Additionally, vaccination can protect children from many preventable diseases like measles. Learn more about vaccine safety. 

There are many causes of autism. Learn about the factors that increase the risk of ASD and shape the type of autism that a child will develop. 

Origin of the “vaccines cause autism” myth 

The hypothesis that autism might be caused by the mumps, measles, and rubella (MMR) vaccine was first proposed in a 1998 study by Wakefield et al. that suggested that vaccinations caused developmental regression in a small group of children. The study only examined 12 children and was later retracted, or taken off the scientific record, because of flawed methodology and scientific misconduct. This study has been widely discredited, and later large-scale studies have disproven its claims, finding no link between the MMR vaccine and autism. 

Research on autism and vaccines 

MMR vaccine and autism  

Autism occurrence by MMR vaccine status among U.S. children with older siblings with and without autism (JAMA, 2015) 

  • This study examined whether the MMR vaccine increases the risk of autism spectrum disorder (ASD) in children, particularly those who have older siblings with ASD and are at higher genetic risk. 
  • Analyzing data from over 95,000 children, researchers found no increased risk of ASD associated with the MMR vaccine at any age, regardless of whether the child had an older sibling with ASD.  

Measles, mumps, rubella vaccination and autism: A nationwide cohort study (Annals of Internal Medicine, 2019) 

  • Analyzing data from over 650,000 children in Denmark, this large-scale study found no link between MMR vaccines and autism. 
  • The risk of autism was not higher in vaccinated children compared to unvaccinated children, even among those with a sibling history of autism or other risk factors.  

A population-based study of measles, mumps, and rubella vaccination and autism (The New England Journal of Medicine, 2002) 

  • In a cohort of over 537,000 children born in Denmark between 1991 and 1998, researchers found no link between MMR vaccination and autism. 
  • The risk of autism in vaccinated children was the same as in unvaccinated children. 
  • Furthermore, there was no association between autism and age at the time of vaccination, time since vaccination or date of vaccination. 

Autism and measles, mumps, and rubella vaccine: no epidemiological evidence for a causal association (The Lancet, 1999) 

  • This study investigated whether the introduction of the MMR vaccine in the U.K. in 1988 was associated with an increase in autism diagnoses or a change in the timing of symptom onset. 
  • Analyzing data from nearly 500 children with autism, researchers found no sudden increase in autism cases after the introduction of the MMR vaccine. 
  • There was no difference in age at diagnosis between vaccinated and unvaccinated children, and no increase of autism onset in the months or years following vaccination. 

MMR-vaccine and regression in autism spectrum disorders: Negative results presented from Japan (Journal of Autism and Developmental Disorders, 2006) 

  • This study examined whether the MMR vaccine was linked to autistic regression in Japan, where the vaccine was only used between 1989 and 1993. 
  • Among 904 children with autism, researchers found no difference in regression rates between those who received MMR and those who did not. 
  • Additionally, the overall incidence of autistic regression remained stable before, during and after MMR vaccine use in Japan. 
Thimerosal exposure 

Association between thimerosal-containing vaccine and autism (JAMA, 2003) 

  • Researchers analyzed data from over 467,000 children in Denmark and found no difference in autism rates between those who received thimerosal-containing vaccines and those who received thimerosal-free versions. 
  • There was also no dose-related effect, meaning higher exposure to ethylmercury (a byproduct of thimerosal, a mercury-based vaccine preservative) did not increase autism risk. 
  • This study provides strong evidence that thimerosal in vaccines does not cause autism, reinforcing the safety of vaccine preservatives. 

Pervasive developmental disorders in Montreal, Quebec, Canada: prevalence and links with immunizations (Pediatrics, 2006) 

  • This study examined whether exposure to thimerosal or the MMR vaccine influenced autism rates in children born in Montreal, Canada between 1987 and 1998. 
  • Autism rates continued to rise even after thimerosal was removed from vaccines in 1996 and when MMR vaccination rates declined. 
  • Additionally, the introduction of a second MMR dose had no effect on autism rates. 
  • The study found that the increasing prevalence of autism was more likely due to changes in diagnostic criteria, increased awareness and better identification rather than vaccines. 

Thimerosal exposure in infants and developmental disorders: A prospective cohort study in the United Kingdom does not support a causal association (Pediatrics, 2004) 

  • Researchers analyzed data from over 14,000 children in the U.K. and found no evidence that thimerosal exposure harmed neurological or psychological development. 
  • In fact, some unadjusted results suggested a slight beneficial effect, though this was likely due to chance. 
  • After adjusting for factors like birth weight and maternal education, no meaningful link between thimerosal and developmental issues was found. 

Autism and thimerosal-containing vaccines: lack of consistent evidence for an association (American Journal of Preventative Medicine, 2003) 

  • This study examined autism trends in California, Sweden and Denmark to see if diagnosis rates increased alongside exposure to thimerosal. 
  • Findings found that autism rates began rising in all three countries in the late 1980s and accelerated in the 1990s, regardless of thimerosal exposure. 
  • While thimerosal use increased in the U.S., it decreased and was eventually eliminated in Sweden and Denmark—yet autism rates continued to rise in all three countries. 

Continuing increases in autism reported to California's developmental services system: mercury in retrograde (Archives in General Psychiatry, 2008) 

  • This study analyzed autism trends in California to determine if autism rates declined after thimerosal was removed from childhood vaccines between 1999 and 2001. 
  • Despite the removal of thimerosal, autism rates continued to rise among children aged 3 to 12 years throughout the study period. 
  • The increase was especially pronounced after 2004, contradicting claims that thimerosal exposure was a major factor in autism diagnoses. 
Immune system response 

Increasing exposure to antibody-stimulating proteins and polysaccharides in vaccines is not associated with risk of autism (The Journal of Pediatrics, 2013) 

  • This study investigated whether the number of antigens (immune system-stimulating components) in vaccines given during the first two years of life was associated with an increased risk of autism. 
  • Analyzing data from 256 children with ASD and 752 children without ASD, researchers found no link between the total number of vaccine antigens received and the likelihood of developing ASD, including specific subtypes like autistic regression. 
  • The study confirms that the cumulative exposure to vaccine antigens in early childhood does not increase the risk of autism. 

Number of antigens in early childhood vaccines and neuropsychological outcomes at age 7–10 years (Pharmacoepidemiology and Drug Safety, 2013) 

  • This study examined whether receiving multiple vaccines in early childhood affects neurodevelopment later in life. 
  • Researchers analyzed data from over 1,000 children who had received thousands of antigens at 7, 12 and 24 months. 
  • They found no negative impact on cognitive abilities, behavior, memory, attention or executive function. 
  • In fact, children with higher vaccine exposure performed slightly better on attention and executive function tests. 
Meta-analyses 

Vaccines are not associated with autism: An evidence-based meta-analysis of case-control and cohort studies (Vaccine, 2014) 

  • Researchers analyzed data from over 1.2 million children across five large cohort studies and five case-control studies. 
  • They found no connection between vaccines and autism, including the MMR vaccine, thimerosal and mercury exposure. 
  • This large-scale meta-review provides strong evidence that vaccines and their ingredients do not cause autism. 

Safety of vaccines used for routine immunization in the United States: An updated systematic review and meta-analysis (Vaccine, 2021) 

  • This systematic review analyzed a vast body of research to assess the safety of vaccines recommended for children, adults and pregnant women in the U.S. 
  • Researchers reviewed 338 studies and found no link between the MMR vaccine and autism. 
  • While the MMR vaccine was associated with a slight increased risk of febrile seizures, this is a known, temporary side effect. 
  • There was also no evidence linking vaccines to conditions like diabetes, intussusception or stillbirth. 
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