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Donald Trump believes vaccines cause autism. Here's the evidence that proves him wrong.

The idea was based on a now thoroughly debunked study, and many other studies have since discredited the idea.

Republican presidential hopefuls Donald Trump and Ben Carson dived into the question of whether vaccines cause autism in the Republican debate Republican debate Wednesday night.

Carson, a pediatric neurosurgeon, pointed out (correctly) that researchers have throughly discredited the notion.

Trump, meanwhile, persisted with his own version of the science. "You take this little beautiful baby," he said, "and you pump — I mean, it looks just like it is meant for a horse, not for a child, and we had so many instances, people that work for me, just the other day, 2 years old, beautiful child went to have the vaccine and came back and a week later got a tremendous fever, got very, very sick, now is autistic."

The claim that vaccine cause autism rose to popularity in 1998, when an esteemed medical journal published a paper with the now infamous conclusion: that the measles, mumps, and rubella vaccine — administered to millions of children across the globe each year — could cause autism.

This study, led by the discredited physician-researcher Andrew Wakefield, has since been thoroughly eviscerated: The Lancet retracted the paper, investigators have described the research as an "elaborate fraud," and Wakefield has lost his medical license.

But public health experts say the false data and erroneous conclusions, while resoundingly rejected in the academic world, still drive some parents' current worries about the MMR shot, and famous folks like Trump continue to push the idea.

Here are six reasons — and many links to further reading — that should remind you why the vaccine-autism claim is bogus.

1) The best available evidence overwhelmingly contradicts the notion that vaccines cause autism

Large-scale studies involving thousands of participants in several countries have failed to establish a link between the MMR vaccine and the mental developmental disorder.

Most recently, the journal Jama looked at nearly 100,000 children who got the shot and their family histories of autism. They found the MMR vaccine was not associated with an increased risk of autism, even with children who had older siblings with the disorder. "These findings indicate no harmful association between MMR vaccine receipt and ASD even among children already at higher risk for ASD," the researchers concluded.

In another of the most thorough studies to date, nearly half a million kids who got the vaccine were compared with some 100,000 who didn't, and there were no differences in the autism rates between the two groups. "This study provides strong evidence against the hypothesis that MMR vaccination causes autism," the authors wrote in the New England Journal of Medicine.

Studies published in The Lancet, The Journal of Pediatric Infectious Diseases, PLOS One, and The Journal of Autism and Developmental Disorders, among others, have also found no association between the vaccine and autism.

2) The MMR vaccine-autism study was just bad science

Wakefield's association between the MMR vaccine and autism was based on a case report involving only 12 children. "Case reports" are detailed stories about particular patients' medical histories, and — because they are basically just stories — they are considered among weakest kinds of medical studies.

Many children have autism and nearly all take the MMR vaccine. Finding in this case that among a group of a dozen children, most of them happen to have both is not at all surprising and in no way proves that the MMR vaccine causes autism. (Wakefield also proposed a link between the vaccine and a new inflammatory bowel syndrome, which has since been called "autistic enterocolitis" and also discredited.)

But don't stop with the retracted study. Again, the totality of the evidence opposes this vaccine-autism theory.

3) Study author Andrew Wakefield manipulated and misrepresented his data

retracted

(The Lancet)

A British investigative journalist, Brian Deer, followed up with the families of each of the 12 kids in the study. He concluded, "No case was free of misreporting or alteration." In other words, Wakefield, the lead author of the original report, manipulated his data. (See the pop-up chart in this report for details.)

In the British Medical Journal, Deer spells out exactly what he found, and it's rather shocking that this study was ever published in the first place. You learn that the parents of many of the kids deny the conclusions in the study; some of the kids Wakefield suggested were diagnosed with autism actually weren't; others who Wakefield suggested were "previously normal" actually had preexisting developmental issues before getting their shots.

4) The paper is based on blood samples Wakefield drew at his kid's birthday party

Even more absurdly, when the General Medical Council (the UK's medical regulator) began to investigate Wakefield, they found that he had paid children at his son's 10th birthday party to donate their blood for his research. That isn't exactly a controlled and ethical setting.

In fact, in deciding to take his UK medical license away, the GMC said Wakefield acted with "callous disregard for the distress and pain the children might suffer."

5) Wakefield filed a patent for an MMR vaccine alternative

Wakefield also had financial conflicts of interest. Among them, while he was discrediting the combination measles-mumps-rubella vaccine and suggesting parents should give their children single shots over a longer period of time, he was conveniently filing patents for single-disease vaccines.

"For the vast majority of children, the MMR vaccine is fine," he said, "but I believe there are sufficient anxieties for a case to be made to administer the three vaccinations separately." He also suggested the long-term safety studies of the MMR shouldn't be trusted.

Brian Deer
's investigation revealed that in June 1997, Wakefield had filed a patent for a supposedly "safer" single measles vaccine. Deer writes, "Although Wakefield denied any such plans, his proposed shot, and a network of companies intended to raise venture capital for purported inventions — including 'a replacement for attenuated viral vaccines,' commercial testing kits and what he claimed to be a possible 'complete cure' for autism — were set out in confidential documents."

6) Wakefield has refused to replicate the paper's findings

At the very bedrock of science is the concept of falsification: a scientist runs a test, gathers his findings, and tries to disprove himself by replicating his experiment in other contexts. Only when that's done can he know that his findings were true.

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Callous Disregard, Wakefield's book. (Image via Amazon.)

Wakefield has never done this. As the editor of the BMJ points out, "Wakefield has been given ample opportunity either to replicate the paper's findings, or to say he was mistaken. He has declined to do either." In 2004, 10 of his co-authors on the original paper retracted it, but Wakefield didn't join them, and he has since continued to push his views, including doing the rounds on the anti-vaxxer speakers' circuit and publishing books.

Wakefield's own website portrays him as an embattled hero: "In the pursuit of possible links between childhood vaccines, intestinal inflammation, and neurologic injury in children, Dr. Wakefield lost his job in the Department of Medicine at London’s Royal Free Hospital, his country, his career, and his medical license." Wakefield even tried to sue the BMJ and Deer, suggesting they were going after him in some sort of vendetta. So far, these lawsuits have gone nowhere.

Watch: Vaccines do not cause autism, they save lives