To be fair, as far as medical hypotheses go, Trump's idea is not completely crazy. Or at least it wouldn't be if this were still 1998.
That year, a well-respected journal published a paper by researcher Andrew Wakefield and 12 of his colleagues linking a standard measles, mumps and rubella vaccine to autism. Despite its tiny sample size of 12 and its speculative conclusions, the study was publicized far and wide -- launching a global movement involving celebrities like Jenny McCarthy, Jim Carrey (and of course Trump) who warned parents to stop vaccinating their children. The result was what public health officials reported was a dangerous drop in MMR vaccinations.
The problem: The study was an elaborate fraud.
Editors of the Lancet, which published the original piece, discovered that Wakefield had been funded by attorneys for parents who were pursuing lawsuits against vaccine companies and that a number of elements of the paper were misreported.
In February 2010, the journal retracted the piece, and in an investigative piece in 2011, in The BMJ found even more shenanigans in the way the study was conducted. Some parents of children in the study reported by Wakefield to have autism said they did not, and others who were listed in the study as having no problems before the vaccine actually had had developmental issues.
Journalist Brian Deer wrote: "No case was free of misreporting or alteration. Taken together ... records cannot be reconciled with what was published, to such devastating effect, in the journal."
Despite these revelations and reassurances from the federal health officials and other experts that vaccines are safe, the public remained fearful.
Much of the alarm came from the case of Hannah Poling -- whose condition after she received five vaccines at 19 months old seemed to confirm every parent's nightmare.
Hannah's parents had described their child as interactive, playful and communicative before she got those shots but reported that after she got the vaccine, she develop problems with language, communication and behavior, features of autism spectrum disorder.
An article in the New England Journal of Medicine described the drama of what happened after her parents sued the Department of Health and Human Services (DHHS) for compensation under the Vaccine Injury Compensation Program (VICP) and won:
On March 6, 2008, the Polings took their case to the public. Standing before a bank of microphones from several major news organizations, Jon Poling said that “the results in this case may well signify a landmark decision with children developing autism following vaccinations.” For years, federal health agencies and professional organizations had reassured the public that vaccines didn't cause autism. Now, with DHHS making this concession in a federal claims court, the government appeared to be saying exactly the opposite. Caught in the middle, clinicians were at a loss to explain the reasoning behind the VICP's decision.
The issue became so controversial back then that dozens of studies were launched to address the question Wakefield posed.
The research, published in top journals including JAMA, the New England Journal of Medicine, the Journal of Pediatric Infectious Diseases and the Journal of Autism and Developmental Disorders, is consistent and confident in its conclusions: There's no link between autism and vaccines.
One of the largest was published in JAMA in April of this year and looked at 96,000 children in the United States and analyzed which ones got the shot and which ones were diagnosed with autism spectrum disorder. They found "no harmful association" between the two.
Another large study, published in the New England Journal of Medicine in 2002, involved a half-million children in Denmark's health registry. Its takeaway: "This study provides strong evidence against the hypothesis that MMR vaccination causes autism."
On Thursday, medical associations and patient advocacy groups decried Trump's remarks as false and potentially dangerous. The American Academy of Pediatrics said that "claims that vaccines are linked to autism, or are unsafe when administered according to the recommended schedule, have been disproven by a robust body of medical literature." Autism Speaks, a science and advocacy group, expressed similar sentiments noting that "extensive research has asked whether there is any link between childhood vaccinations and autism."
"The results of this research are clear: Vaccines do not cause autism," the organization said in a statement.
It cites numerous studies, including a 2013 study that looked at the substances in vaccines that cause the body’s immune system to produce disease-fighting antibodies, showed that the total amount from vaccines received was the same between children with autism and those without.
The CDC said it has looked specifically into thimerosal, a mercury-based preservative used in multidose vials of vaccines that has been a source of concern among those who believe in an autism-vaccine link, and found no link. A review in 2004 by the Institute of Medicine concluded that "the evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism."
Today, most scientists believe that autism there is no single cause of autism, but that genetics and abnormalities in brain structure or function may play a role.
For more health news, you can sign up for our weekly newsletter here.