A prominent anti-vaccine crusader said on Tuesday that President-elect Donald J. Trump had asked him to lead a new government commission on vaccine safety and scientific integrity — a possibility that spread alarm among medical experts that Mr. Trump could be giving credence to debunked conspiracy theories about the dangers of immunizations.
The vaccine skeptic, Robert F. Kennedy Jr., a nephew of former President John F. Kennedy, said that Mr. Trump, who has repeatedly embraced discredited links between vaccines and autism, had asked him to lead the commission during a meeting with the president-elect at Trump Tower on Tuesday.
A few hours after Mr. Kennedy told reporters about the meeting, Hope Hicks, a spokeswoman for Mr. Trump, said that the president-elect was “exploring the possibility of forming a committee on autism, which affects so many families.” But Ms. Hicks added that no final decisions had been made.
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Medical experts have for years rejected claims of ties between childhood vaccines and conditions like autism. They warned that Mr. Trump’s actions would endanger children by confusing parents about the vital need to get them vaccinated.
“It gives it a quasi-legitimacy that I frankly find frightening,” said William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University. He said Mr. Trump and Mr. Kennedy were being fooled by “long-discredited” theories about vaccines.
“This is going to be a sad struggle as we try to protect as many children as possible,” Dr. Schaffner said.
Told of the meeting between Mr. Trump and Mr. Kennedy, Dr. Carrie L. Byington, the chairwoman of the committee on infectious diseases for the American Academy of Pediatrics and dean of the College of Medicine at Texas A&M University, urged parents to make sure their children get immunized.
“Vaccines are safe, and vaccines save the lives of children and adults every day,” said Ms. Byington, whose committee released a report in August defending vaccines. “The science of vaccines is well established, and the safety of vaccines is well established.”
Both Mr. Trump and Mr. Kennedy have described themselves as “pro-vaccine.” But they have repeatedly expressed concerns about what they claim is a link between vaccines and the development of autism. At a Republican presidential debate in September 2015, Mr. Trump described knowing people personally who had seen a cause and effect.
“Autism has become an epidemic,” Mr. Trump said in the debate. “Twenty-five years ago, 35 years ago, you look at the statistics, not even close. It has gotten totally out of control.”
“I am totally in favor of vaccines,” he added. “But I want smaller doses over a longer period of time. Same exact amount, but you take this little beautiful baby, and you pump — I mean, it looks just like it’s meant for a horse, not for a child, and we’ve had so many instances, people that work for me.”
Mr. Trump has also repeatedly used Twitter to spread his concerns about the safety of vaccines. In particular, he has often raised doubts about giving children vaccines in a single large dose rather than several smaller ones.
“Healthy young child goes to doctor, gets pumped with massive shot of many vaccines, doesn’t feel good and changes,” he wrote in March 2014. “AUTISM. Many such cases!”
Mr. Kennedy said in an interview that he had received word from people connected to Mr. Trump’s transition team that there might be a vaccine initiative and inquiring about whether he would be interested in participating.
He said he had spoken to Mr. Trump by telephone ahead of Tuesday’s meeting, which was also attended by Stephen K. Bannon, the president-elect’s chief strategist; Kellyanne Conway, Mr. Trump’s counselor; and Vice President-elect Mike Pence.
“He told me he has a number of friends who are affected,” Mr. Kennedy said of the conversation with the president-elect, describing 2-year-old children who had received “a battery” of vaccines and then exhibited a high fever followed by changes in behavior. “They all have the same story.”
Mr. Kennedy said Mr. Trump “believes in those anecdotal stories” about the dangers of vaccines. He said the president-elect “says if you have enough anecdotal stories saying the exact same thing, that you can’t dismiss the validity.”
He said a commission would probably last a year and be made up of “a mix of science people and other people hopefully from the media and other prominent Americans who hopefully don’t have a position going in.”
Mr. Kennedy wrote a book in 2014, “Thimerosal: Let the Science Speak,” arguing that a mercury-based preservative called thimerosal, found in vaccines, was responsible for causing autism in children. On his website, he accuses the Centers for Disease Control and Prevention of being a “cesspool of corruption, mismanagement and dysfunction” that has led the agency to overlook the affects of vaccines.
“With the research, regulatory and policy-making agencies captured, the courts closed to the public, the lawyers disarmed, the politicians on retainer and the media subverted, there is no one left to stand between a greedy industry and vulnerable children, except parents,” Mr. Kennedy wrote.
Such claims have been fueled over the past several years by social media, sometimes embraced by actors or fringe political candidates, often to the dismay of public health officials, who say the commentary is contrary to the facts.
The actress Kirstie Alley has written on Twitter that not all vaccines are harmless. Erin Brockovich, an environmental activist, wrote on the site in 2015, “I’ll be damned if any government is going to tell me what to put in my or kids’ bodies.” In 2011, Michele Bachmann, then a member of Congress from Minnesota and a Republican presidential candidate, called the vaccine to prevent cervical cancer “dangerous.”
Despite those testimonials, the idea of a link between vaccines and autism has been widely and repeatedly discredited by scientific studies over many years.
The conspiracy theory is largely based on a small study of 12 children that was published in the British medical journal The Lancet in 1998 by Andrew Wakefield, who suggested that the measles–mumps–rubella vaccine could cause autism.
After a wave of studies debunking the study, it was retracted by The Lancet in 2010. And Britain’s General Medical Council, citing ethical violations and failure to disclose financial conflicts of interest, revoked Mr. Wakefield’s medical license.
Scientific evidence has repeatedly shown the vaccine, given to children between 12 and 15 months of age, to be safe, as well as highly effective at preventing those diseases.
And refusal to immunize children can have serious consequences — for those children and for others who might then be vulnerable to infection by the circulating disease. In recent years, when parents have declined to vaccinate their children, serious outbreaks of measles have erupted, including one at Disneyland.
Dr. Bynum said that if the commission were formed, she and other representatives from medical groups would want to be part of it “to represent vaccine safety.” Dr. Schaffner said he was concerned that Mr. Trump’s actions would lead people to doubt the advice they get from their family doctors.
“What they will say is: ‘I don’t know who’s right here.’ And they will withhold vaccines,” he said. “All of those things will leave more children, more susceptible to a variety of diseases. That breaks my heart.”