How scientists figured that out

In the late 1990s, a theory gripped parents around the world: What if childhood vaccines—particularly the combined measles-mumps-rubella vaccine—cause autism? Nearly three decades later, the debunked theory has gained renewed prominence in parallel with the rise of Robert F. Kennedy Jr., a long-time anti-vaccine activist who now serves as the U.S. health secretary.

Scientists, medical providers and medical researchers are clear on this: Vaccines do not cause autism.

To those tuning into the vaccine debate now, it can feel like scientists are dismissing the theory without considering it. But there’s a reason. There has already been a mountain of research investigating vaccines and debunking any connection with autism.

Scientists have reached a consensus on this one. Here’s how.

Where did the ‘vaccines cause autism’ theory come from?

The idea that vaccines cause autism, or at least the popularity of that idea, can be traced to a 1998 research paper.

In that paper, then-physician Andrew Wakefield and his co-authors hypothesized that the measles-mumps-rubella vaccine could trigger a combination of gastrointestinal problems and neurodevelopmental “regression” consistent with autism. Wakefield, who is British, published the article in the well-respected scientific journal The Lancet. The paper had a dozen co-authors.

(That paper, and Wakefield himself, have since been resoundingly discredited. But let’s take this in chronological order.)

The paper, in its initial form, had numerous red flags, including a small sample size of 12 children. Additionally, the paper itself did not claim to actually prove the connection.

“We did not prove an association between measles, mumps and rubella vaccine and the syndrome described,” the article says. “Virological studies are underway that may help to resolve this issue.”

Even with the original paper’s caveats and limitations, the theory captured public attention and was simplified to the idea that vaccines cause autism. Researchers and medical professionals have pointed to the paper as one major cause of dropping childhood vaccination rates.

The scientific community took note of the paper, too, and responded in the way that science should: by testing the hypothesis again and again.

Did other scientists ‘really’ look into Wakefield’s idea?

In the years after the Wakefield paper, researchers around the world embarked on studies to look for any link between the MMR vaccine and autism.

David Amaral—the founding research director at the MIND Institute at UC Davis, an autism research center—has worked in autism research since the late 1990s, around the same time as the Wakefield paper’s publication. He remembers the flurry of research triggered by the vaccine theory.

There was a 1999 epidemiological study in the U.K., published in the same journal that published Wakefield’s paper the year before. There was a 2001 study in California. There was a 2002 study of more than half a million children in Denmark. There was another 2002 study of more than half a million children, this time in Finland.

These four examples are just a sampling of the studies on vaccines and neurodevelopmental disorders. There have been many other studies, too.

The Autism and Science Foundation has compiled a lengthy list that includes at least 11 additional research studies and analyses related to vaccines and autism.

Dr. Jake Scott, a Stanford Medicine professor and an infectious disease specialist, has spearheaded a compilation of randomized controlled trials—which are often cited as the gold standard of research—related to vaccines and vaccine safety. That spreadsheet, Scott wrote in early September federal testimony, has cataloged more than 1,700 studies.

“If vaccines caused a wave of chronic disease, our safety systems—which can detect 1-in-a-million events—would have seen it,” Scott wrote in his testimony. “They haven’t.”

What did those other studies find?

A number of the studies conducted after the Wakefield paper focused on rates of autism among children who had been vaccinated and children who hadn’t. If the MMR vaccine—or another vaccine—did cause or increase the risk of autism, then there should be higher rates of autism among vaccinated children.

This was a reality that Wakefield and his co-authors directly addressed in their original paper.

“If there is a causal link between the measles, mumps, and rubella vaccine and this syndrome,” the paper says, “a rising incidence might be anticipated after the introduction of this vaccine in the UK in 1988.”

Scientists and researchers had the same thought.

“If there was a link between the vaccines, you should at least see some signal,” Amaral said. “You should see some evidence of it.”

But, in follow-up study after follow-up study, researchers found no such evidence.

The 1999 UK study authors wrote that their “analyses do not support a causal association between the MMR vaccine and autism.”

“This study provides strong evidence against the hypothesis that MMR vaccination causes autism,” said the authors of the massive Denmark study.

And on and on.

The studies repeatedly came back with the same conclusion: No evidence of a link between the measles-mumps-rubella vaccine and autism. That matters because, in the scientific world, a single study cannot prove anything.

When a researcher finds a new or important link, other researchers set off on the task of replicating that finding. This is to ensure that the link really does exist, that it wasn’t a mistake by the original researcher, a poorly designed study or simply a fluke. (Spoiler alert: That scientific process also aims to root out deliberate research fraud.)

It is only when a finding can be repeated by other researchers that the scientific community reaches “consensus” and agrees that a hypothesis does, indeed, appear to be true.

When other researchers set out to validate—or replicate—Wakefield’s study, they couldn’t. It was a death knell for Wakefield’s hypothesis.

Dr. Jason Terk, a pediatrician in North Texas, started practicing in 1998, the same year the Wakefield paper was published.

“In good faith, the medical community tried to answer the question and it was a very clear answer,” Terk said, “that there was no evidence to support the connection that was raised by this paper.”

Even after extensive research, scientists tend to be hesitant to frame anything as an absolute. If there’s a hair’s width opening for a possible other explanation, researchers tend to hedge their statements, to leave room for that extraordinarily unlikely other explanation.

In the case of vaccines and autism, though, researchers have come as close to absolute proof as they can.

“There is literally no evidence that vaccines cause autism,” said Katherine Meltzoff, a University of California, Riverside professor and autism researcher. “It’s been studied and restudied and restudied almost infinite times since the initial, fraudulent 1998 paper came out.”

What happened to Wakefield and his paper?

As evidence mounted against Wakefield’s hypothesis and his 1998 paper, The Lancet investigated allegations that the paper’s results may have been fabricated or otherwise manipulated.

In 2004, 10 of the 12 co-authors retracted the interpretation of the original data in the paper.

Six years after that, the journal retracted the entire article, citing “false” statements in the study, including in how the authors recruited patients.

The British medical board, called the General Medical Council, looked into the situation, too. In a report, the council detailed numerous instances of serious misconduct, including that Wakefield had not disclosed that he was being paid by an organization that was building a lawsuit against vaccine manufacturers. The council ultimately revoked Wakefield’s medical license.

Subsequent journalistic articles, published in the medical journal The BMJ, found Wakefield and his co-authors had deliberately falsified the data in the original paper.

The physician and his research paper were discredited, thoroughly disgraced by all of the levers of scientific accountability.

However, the full retraction did not take place until 2010. That was 12 years after the original article’s publication. It was, in many ways, too late.

Wakefield’s idea has continued to spread, fueling vaccine fear and hesitancy around the world.

A note on thimerosal, and other vaccine-related theories

Variations of Wakefield’s theory have sometimes grabbed the public attention, too, focusing on different aspects of vaccines.

Among the most well-known was the theory that it was not the MMR vaccine itself that caused autism, but a mercury-based preservative called thimerosal.

This, too, has been studied, and no link has been found.

Still, thimerosal has been removed from all routine childhood vaccines in the United States, or left only in barely detectable amounts. If thimerosal did cause autism, then researchers could have expected to see a drop in autism rates after the preservative was removed. However, a study in California found autism rates did not drop after the removal of thimerosal. Other studies similarly found no link between thimerosal and autism.

Other vaccine-related theories have popped up, too, variations on the Wakefield hypothesis. But researchers say there’s a blanket reason not to worry about a connection with autism: The timing doesn’t make sense.

Children can show signs of autism in infancy, and researchers have found evidence of brain abnormalities beginning in the womb. While children may not receive diagnoses until they’re toddlers or a little older, those early signs indicate that autism was already there before the official diagnosis.

That means, Amaral said, that vaccines can’t be the cause of autism. The timing doesn’t add up.

“I know of little to no evidence that autism can be caused by anything happening postnatally,” Amaral said. “There just isn’t any evidence for that.”

Dr. Peter Hotez is a long-time vaccine researcher based in Texas and the father of a child with autism. No matter the version of the theory, Hotez is confident there will not prove to be any connection between vaccines and autism.

It comes down to a “lack of plausibility,” Hotez said. “All of this occurs in pregnancy, through the action of autism genes.”

If it’s not vaccines, then what does cause autism?

Autism researchers have been working for decades to figure out what causes autism.

Researchers still don’t have all the answers, but there are a lot of things they do know. Amaral highlighted two things in particular.

First, there’s a strong genetic component to autism. Researchers have identified more than 200 genes that are linked to autism.

It’ll be difficult for researchers to pinpoint all of the genetic influences because of the complexity of genetics. Even seemingly simple genetic equations can be incredibly complex. For example, researchers set off years ago to find all the genes that influence a person’s height. They ultimately found more than 12,000 genetic variants.

“And autism is probably going to be more complicated,” Amaral said.

Secondly, there are environmental factors that interact with the genetic factors and increase the likelihood a child will have autism. Importantly, Amaral said, researchers know any environmental factors connected to autism are already at play by the time a child is born. Researchers know this even without knowing the full array of environmental factors at play, because signs of autism can be detected in very young infants and even in fetuses.

“The evidence is overwhelming that autism starts prenatally,” Amaral said.

The experts said there are a number of environmental factors that may play a role in the development of autism. Advanced paternal age, maternal fever during pregnancy and pesticide exposure during pregnancy—which is an elevated risk for certain populations, such as farm workers—have all been linked to an increased likelihood a child will have autism.

In recent months, the presidential administration has elevated a theory that autism may be caused by maternal use of Tylenol during pregnancy. Researchers and experts say that theory is not supported by evidence, and they do not believe Tylenol is one of the environmental factors of autism.

Approaching parents with respect

Autism researchers and other experts said they can understand why some parents latched onto the idea that vaccines could cause autism.

While signals of autism show up in infancy, the most obvious signs often don’t show up until children are a little older, around the time most children begin to develop more social skills. Autism diagnosis, then, can line up approximately with the time of some vaccinations, including the measles-mumps-rubella vaccine.

And because there isn’t a clear-cut single answer to what does cause autism, it can be easier to point the finger at something more concrete, even though it isn’t true.

“For years, we didn’t know what caused autism,” said Alycia Halladay, the chief science officer at the Autism Science Foundation, “and so vaccines were an easy thing to blame.”

With all of the work that’s been done to investigate any connection between vaccines and autism, it is resoundingly clear there is no link between the two.

With so much news and “static” about vaccines, though, Terk said he understands why parents may have questions or concerns. The pediatrician said he focuses on validation and respect for those parents, while answering their questions with science and facts.

“I remind them that we—the parents and me, the pediatrician—are on the same team,” Terk said. “We both want the same thing, which is to support the health and well-being of their child.”

Medical professionals emphasized that parents should talk to their child’s pediatrician about any vaccine concerns they have. Vaccines are safe and they don’t cause autism, but it’s still okay to ask medical professionals for more information.

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