Robert F. Kennedy Jr. speaks with members of the press at Trump Tower in Manhattan. (Stephanie Keith/Reuters)

First things first: Vaccines do not cause autism.

So say the Centers for Disease Control and Prevention, the World Health Organization, the National Academies of Sciences, Engineering and Medicine, and the American Academy of Pediatrics, along with dozens of studies published in prestigious, peer-reviewed journals. The scientific consensus on vaccines and autism is thorough and solid: There is no evidence of a connection.

This is not new news. But it bears repeating now that Robert F. Kennedy Jr. says President-elect Donald Trump wants him to chair a new commission on vaccine safety. Because unlike thousands of doctors and scientists, Kennedy — a lawyer who does not have a medical degree — is not convinced.

The debunked claim that there is a causal relationship between vaccines and autism largely stems from the late 1990s. At the time, autism diagnoses had been increasing, and doctors didn't know why.

In 1998, British researcher Andrew Wakefield published his infamous paper linking autism to the vaccine for measles, mumps and rubella (MMR). Prompted in part by Wakefield's research, U.S. agencies investigated a mercury-based vaccine additive called thimerosal, which was widely used as an antifungal and antiseptic agent. Because of new research on the dangers of mercury poisoning, the government wanted to ensure that thimerosal didn't pose a risk to children who were being vaccinated.

The Food and Drug Administration, along with the National Institutes of Health, the CDC, the AAP and several other agencies, concluded that there was no evidence that thimerosal caused any harm. But the additive was removed from most vaccines anyway, as a precautionary measure.

Meanwhile, research linking the MMR vaccine to autism was unraveling. Evidence emerged that Wakefield had been paid by attorneys for parents who were suing MMR manufacturers and that Wakefield's data were fraudulent. The Lancet retracted his study in February 2010. That year, Wakefield was found guilty of professional misconduct by Britain's General Medical Council and his license was revoked.

But doubt and fear had been sown. The anti-vaccine movement grew and grew. In 2015, nearly 200 Americans were sickened with measles — a disease that was declared eradicated 15 years earlier — as a consequence of parents not vaccinating their kids, according to the CDC.

The consensus of the medical and scientific establishment hasn't deterred vaccine skeptics, many of whom say that the government must be covering up the risk. This conspiracy theory found fertile ground in Kennedy.

In 2005, Kennedy published a splashy article in Rolling Stone and Salon.com titled “Deadly Immunity.” He enumerated the supposed dangers of thimerosal, which by then was used in few vaccines other than the flu shot, and alleged that government researchers were hiding evidence that it could cause autism and other neurological disorders. “Our public-health authorities knowingly allowed the pharmaceutical industry to poison an entire generation of American children,” he claimed.

The article contained several factual errors, and Salon.com eventually retracted it. Rolling Stone left the piece on its site but had to publish more than half a dozen corrections. A decade later, the evidence continues to reaffirm the conclusion that vaccines have no connection of any kind to autism.

Kennedy has held on to his claim. He expanded his Rolling Stone article into a book published in 2014, though collaborator Mark Hyman, a physician, softened some of his rhetoric. In 2013, Kennedy told Slate that the CDC has been lying to the public. In 2015, while promoting the anti-vaccine film “Trace Amounts,” he told audiences, “They get the shot, that night they have a fever of a hundred and three, they go to sleep, and three months later their brain is gone. This is a holocaust, what this is doing to our country.”

Kennedy says he is “pro-vaccine,” insisting that he just wants to ensure vaccines are “as safe as they possibly can be.”

But medical professionals argue that the seeds of doubt that Kennedy and others like him have planted can have dangerous consequences, weakening parents' faith in something the medical establishment knows is vital.

“The speculative vaccination-autism connection decreased parental confidence in public health vaccination programs and created a public health crisis in England and questions about vaccine safety in North America,” immunologist Dennis Flaherty wrote in 2011. “The alleged autism-vaccine connection is, perhaps, the most damaging medical hoax of the last 100 years.”

Soon after news of Kennedy's appointment broke, the AAP issued a statement from its president, Fernando Stein, and vice president, Karen Remley, affirming the “safety and importance of vaccines.” They emphasized the risks of delaying vaccinations, something that Trump said he was in favor of during a 2015 Republican primary debate. “I want smaller doses over a longer period of time,” Trump said.

The AAP's response was blunt: “Vaccines are safe. Vaccines are effective. Vaccines save lives,” Stein and Remley wrote. “We stand ready to work with the White House and the federal government to share the extensive scientific evidence demonstrating the safety of vaccines, including the recommended schedule.” The real danger, they point out, lies in refusing or delaying vaccines.