Vaccines do not cause autism. You’ve almost certainly read that before — probably hundreds of times. But many people do not believe it, perhaps because too often it is repeated without a real explanation of how we know that.
So here is an attempt to offer that explanation.
Of course, the issue is in the news again because Robert f. Kennedy Jr., President Trump’s nominee to lead the Department of Health and Human Services, has spent a career arguing that vaccines do cause autism, as well as other disorders. During Kennedy’s testimony before a Senate committee last week, Sen. Bill Cassidy, a Louisiana Republican who is also a physician, pushed him to “reassure mothers unequivocally and without qualification” that vaccines do not cause autism.
“If the data is there, I will absolutely do that,” RfK Jr. said. “Not only will I do that, but I will apologize for any statements that misled people otherwise.”
The data are there — and have been for years. In fact, here they are.
There is not just one argument that has been advanced about whether vaccines cause autism (technically autism spectrum disorder). There are at least three:
- That autism is caused by doses of toxins used as preservatives or adjuvants in vaccines, with the alleged toxin implicated most commonly being thimerosal, a preservative that was removed from most vaccines because of concerns, expressed without evidence, that it might be responsible for the upsurge of cases in autism.
- That autism can be caused by an immune response to the measles-mumps-rubella vaccine (MMR); this was the claim famously made by the disgraced scientist Andrew Wakefield in 1998, whose study was retracted by the journal The Lancet.
- That autism and other conditions result not from any single vaccine but from the increasing number of vaccines that children receive.
Scientists have responded to each of these arguments with many separate studies that both examined the mechanisms by which vaccines could allegedly cause autism and the broader question of whether children were more likely to develop autism if they had received vaccines. Again and again, the answer has been that they don’t.
Thimerosal
The thimerosal theory emerged in part because of a review conducted by the food and Drug Administration. Thimerosal is a molecule that contains mercury, and it was known that some mercury-containing chemicals can hurt babies’ brain development. (Thimerosal is not among them.) The fDA found that as the number of vaccines given to children had increased, thimerosal dosage had crept higher — high enough that the federal health agencies, the American Academy of Pediatrics, and vaccine manufacturers all agreed to remove or dramatically reduce the levels of thimerosal in vaccines more than 20 years ago as a precautionary measure, although there was no evidence the chemical caused harm.
After this change, autism rates rose unabated, and scientists looking at observational data put a fine point on that. In 2004, researchers in Denmark, which keeps meticulous and centralized health records, published a study of all children vaccinated there between 1971 and 2000.
“There was no trend toward an increase in the incidence of autism during that period when thimerosal was used in Denmark,” the researchers wrote. And the increases continued after thimerosal was discontinued.
Other observational studies also showed no relationship: One study of 14,000 babies in the United Kingdom found that vaccines, if anything, were associated with fewer cases of autism; another, of 103,403 British children, found a possible relationship between vaccines and tics, a symptom of some people with autism but no other association with the condition. (This finding has not been replicated by other studies.) Another study gave a battery of neuropsychological tests to children and found the results did not support the idea that thimerosal caused any health conditions.
Observational studies are limited in what they can detect; they are liable to show risks and benefits where none exist. But in a case where a treatment was causing a massive upsurge in cases of a disorder, one would expect that children who received the intervention would be more likely to develop the disorder. And that has not happened.
MMR
Just as the controversy over thimerosal was brewing, vaccine critics were leveling the same kind of allegations about the measles, mumps, and rubella vaccine.
The measles vaccine, in particular, has had a huge impact. When the first measles shot was introduced in 1963, there were 500,000 cases of the disease in the U.S. annually, resulting in 50,000 hospitalizations, 1,000 cases of brain swelling, and 500 deaths each year. In 2010, before vaccination rates began to fall, there were just 63 cases of measles in the U.S., all the result of imported measles virus. (The country was declared measles-free in 2000.) The measles vaccine was combined with the vaccines for mumps, a painful childhood illness, and rubella, a virus that causes brain damage, in 1971 so kids could get all the immunizations in two-shot series.
It’s crystal clear where the MMR charge arose: an infamous 1998 publication in The Lancet authored by Wakefield, a British doctor, and his colleagues. Wakefield’s paper, even without controversy, is surprisingly scant on details: It proposed a new disorder based on a consecutive series of 12 children who lost acquired skills and seemed to regress and developed abdominal pain. for eight of the children, Wakefield wrote, parents had noticed this after their children had been administered the MMR vaccine.
following a six-year investigation by the British journalist Brian Deer, The Lancet retracted the paper in february 2010. Wakefield was accused of filing for a patent on his own version of the measles vaccine, and of not getting permission to do invasive tests in children. Later that year, Wakefield’s license to practice medicine in Britain was stripped.
The work was discredited in part because other scientists, mostly using money from their respective governments, did study after study — at least 20 of them — demonstrating that the mechanisms Wakefield had proposed didn’t make much sense. But on a bigger scale, again, they showed that children who received MMR were not more likely to develop autism.
Quickly after the paper appeared, researchers in finland published a study saying that there was no sign of the syndrome Wakefield described in the country’s monitoring of children who had received the MMR vaccine over the course of 14 years. A 1999 study of autistic children in the U.K. found that there was no relationship between vaccines and an autism diagnosis, regardless of whether the children were vaccinated, or how early they were vaccinated. It also found that autism diagnoses didn’t just occur shortly after MMR vaccination. A 2001 study showed no correlation between when children in California got the MMR shot and whether they developed autism. A study in Atlanta produced similar results. A 2010 roundup of studies on the MMR shot lists six major studies from the U.K., two from the U.S., three from finland, and one from Denmark.
Again, one of the most conclusive pieces of evidence came from Denmark’s health data system, which showed that the risk of an autism diagnosis was lower among children who received the MMR vaccine. It was published in the New England Journal of Medicine.
The ‘too many doses’ theory
Rather than helping to quiet unfounded allegations about vaccines and autism, all of the studies and public dialogue has had the opposite effect: It gave some people the impression there are major, legitimate concerns on the issue. The idea makes intuitive sense to many people, in part because children develop autism spectrum disorders at about the same age as they receive vaccines.
But the theory also defies logic. The MMR vaccine never contained thimerosal, even before thimerosal was removed from most vaccines. Kennedy has at times backed both the theory about thimerosal and the theory about the MMR shot. He edited a book on thimerosal in 2014; Samoan officials have said he may have worsened a measles outbreak that led to 83 deaths in 2019 by encouraging anti-MMR rhetoric.
A third, completely different idea has become popular in arguing that vaccines cause autism: that receiving too many shots of all types somehow results in an immune reaction that causes the disorder. This notion was articulated in 2015 by Donald Trump.
“I’ve gotten many letters from people fighting autism thanking me for stating how dangerous 38 vaccines on a baby/toddler under 24 months are,” Trump said in a social media post at the time. “It is totally insane — a baby cannot handle such tremendous trauma.”
Researchers have had two responses to this allegation: first, data don’t indicate that vaccines increase the risk that children will contract other infections. Second, vaccines have become much more targeted over time, often involving fewer antigens to stimulate the immune system than earlier versions. Vaccines for pneumococcus, whooping cough, and other diseases now often contain only sugar molecules or proteins from the coat of a virus in order to produce an immune response. By this measure, children get more shots, but they contain fewer antigens.
But again, the way to test this is not through arguments about how the body works, but by looking at children who are vaccinated more and seeing whether they are more likely to develop autism. It’s possible to measure how many antibodies generated by different vaccines children have, and a 2013 study by researchers at the Centers for Disease Control and Prevention found there was no relationship between a measure of these vaccine antibodies and risk of an autism diagnosis.
People who are concerned about vaccine safety often want new randomized controlled trials of vaccines. (Approved vaccines have generally been tested against a placebo, another vaccine, or a placebo that contains other ingredients in the vaccine but not the antigen that provokes an immune response.) But such studies, though the medical gold standard, are hard to do: Doctors already consider the current vaccines the standard of care, raising ethical questions. It’s also hard to imagine parents who either doubt or believe in vaccination but are willing to have their child’s vaccination status decided by a random number generator.
There are other explanations for why autism cases have increased. One is that mental health diagnoses are more socially palatable than they once were; criteria for the diagnosis of autism spectrum disorder have also been consciously widened by experts. But there has also been research showing that there are many genetic factors associated with autism, and that older parental age can play a role in increasing risk.
It’s also worth noting that although public health officials would argue that every element of the vaccine schedule is important, the MMR vaccine in particular is close to the top of the list. One indication: Other countries embrace it, too. In an opinion article, the oncologist and medical contrarian Vinay Prasad — who has defended Kennedy — has been among those to note that other nations use the MMR shot, even if their schedules are somewhat different.
“If RfK Jr. uses his perch as HHS secretary to discourage parents from getting their children inoculated with the MMR vaccine, severe negative repercussions could result, including measles outbreaks and childhood deaths,” Prasad wrote. “This is not a good policy.”
