The claimed mechanisms have changed over time, in response to evidence refuting each in turn. Anti-vaccine groups claim that specific vaccine ingredients can cause autism. Some of the most frequently mentioned ones are
thiomersal, aluminium adjuvants and
formaldehyde.
MMR vaccine The idea of a link between the
MMR vaccine and autism came to prominence after the publication of
a paper by
Andrew Wakefield and others in
The Lancet in 1998. This paper, which was
retracted in 2010 and whose publication led to Wakefield being
struck off the
British medical register, has been described as "the most damaging medical hoax of the last 100 years". Wakefield's primary claim was that he had isolated evidence of vaccine-strain measles virus RNA in the intestines of autistic children, leading to a condition he termed
autistic enterocolitis (a condition never recognised or adopted by the scientific community). This finding was later shown to be due to errors made by the laboratory where the
polymerase chain reaction (PCR) tests were performed. In 2009
The Sunday Times reported that Wakefield had manipulated patient data and misreported results in his 1998 paper, thus falsifying a link with autism. A 2011 article in the
British Medical Journal describes the way in which Wakefield manipulated the data in his study in order to arrive at his predetermined conclusion. An accompanying editorial in the same journal described Wakefield's work as an "elaborate
fraud" which led to lower vaccination rates, putting hundreds of thousands of children at risk and diverting funding and other resources from research into the true cause of autism. On 12 February 2009 a special court convened in the United States to review claims under its
National Vaccine Injury Compensation Program ruled that parents of autistic children are not entitled to compensation in their contention that certain vaccines caused their children to develop autism. The
Centers for Disease Control and Prevention (CDC), the IOM of the
United States National Academy of Sciences, have all concluded that there is no link between the MMR vaccine and autism. A systematic review by the
Cochrane Library concluded that there is no credible link between the MMR vaccine and autism, that the MMR vaccine has prevented diseases that still carry a heavy burden of death and complications, that the lack of confidence in the MMR vaccine has damaged public health, and that the design and reporting of safety outcomes in MMR vaccine studies are largely inadequate. Further, an epidemiology study concluded that even children labelled high risk for autism, due to an older autistic sibling, that received the MMR vaccine resulted in no causal connection between the vaccine and autism or the increased risk of being diagnosed with autism. The assumption that MMR vaccines cause autism is not isolated to the United States. A seven-year study was done in Denmark from 1991 to 1998 following children who received the MMR vaccine. The results of the study found that when comparing the vaccinated children to the unvaccinated children, the risk of autism in the vaccinated group was 0.92. Thiomersal contains
ethylmercury, a mercury compound which is related to, but significantly less toxic than, the
neurotoxic pollutant
methylmercury. Despite decades of safe use, public campaigns prompted the CDC and the
American Academy of Pediatrics (AAP) to request vaccine makers to remove thiomersal from vaccines as quickly as possible on the
precautionary principle. Thiomersal is now absent from all common United States and
European Union vaccines, except for some preparations of
influenza vaccine. Trace amounts remain in some vaccines due to production processes, at an approximate maximum of 1 microgramme, around 15% of the average daily mercury intake in the US for adults and 2.5% of the daily level considered tolerable by the
World Health Organization (WHO). The action engendered concern thiomersal could have been responsible for autism. There is no accepted scientific evidence that exposure to thiomersal is a factor in causing autism. A study by the CDC exploring mercury poisoning in vaccines concluded no signs of poisoning were present. Despite this, starting in 2000, parents in the United States pursued legal compensation from a federal fund arguing that thiomersal caused autism in their children. A 2004
Institute of Medicine (IOM) committee favored rejecting any causal relationship between autism and vaccines containing thiomersal and rulings from the
vaccine court in three test claims in 2010 established the precedent that thiomersal is not considered a cause of autism.
Aluminium adjuvants As mercury compounds in vaccines have been definitively ruled out as a cause of autism, some anti-vaccine activists propose
aluminium adjuvants as the cause of autism. Aluminium adjuvants simulate immune receptors and cause a strengthened response to the antigen in a way that is natural to the body. Aluminium adjuvants can be used in the form of soluble salts, alumina, and hydroxide. These are mainly published in
predatory open access journals, where
peer-review is virtually non-existent. Work conducted by
Christopher Shaw,
Christopher Exley and Lucija Tomljenovic has been funded by the anti-vaccination
Dwoskin Family Foundation. The work published by Shaw et al. has been discredited by the
World Health Organization. A review study published in the
open-access journal
Toxics suggests a link between early aluminium adjuvant exposure and autism; and concludes that there is a lack of fundamental scientific data demonstrating that aluminium adjuvants are safe.
Formaldehyde Formaldehyde is another assumed link between vaccines and autism. Even though the assumption still circles around, formaldehyde has been used safely in the diphtheria vaccines to detoxify the bacteria used to make the vaccine. Another way it can be used is to inactivate the disease to be used in the vaccine. The
Children's Hospital of Philadelphia Vaccine Education Center compiled a list of vaccines recommended to children throughout history. They found that from 1985-1994 the recommended number of vaccines was eight. The schedule for 2011 to 2020 revealed the recommended number of vaccines was fourteen. There have been multiple cases reported similar to this one, which led to the belief that vaccine overload caused autism. However, scientific studies show that vaccines do not overwhelm the immune system. Other scientific findings support the idea that vaccinations, and even multiple concurrent vaccinations, do not weaken the immune system and evidence that autism has any immune-mediated
pathophysiology has still not been found.
COVID-19 A study, initially published in 2024, claimed that the
COVID-19 vaccine could generate autism traits in male offsprings of rats. The study has been criticised early on by scientists for its dubious methodology, and has been, in the summer of 2025, retracted on the grounds of methodological concerns. == Celebrity involvement and social media ==