The CNN moderator laid out the question for Ben Carson like a baseball on a tee, just waiting to be crushed.

“Dr. Carson, Donald Trump has publicly and repeatedly linked vaccines, childhood vaccines, to autism, which, as you know, the medical community adamantly disputes,” Jake Tapper said. “You’re a pediatric neurosurgeon. Should Mr. Trump stop saying this?”

For months, Carson has touted his medical expertise while on the campaign trail. And in the weeks since the first debate, the famed surgeon has risen in the polls as a milder-mannered, more rational alternative to Trump.

Now was his chance for a home run; a big hit as swift and incisive as any surgical operation.

Instead, Carson bunted.

“Well, let me put it this way,” he began hesitantly. “There has — there have been numerous studies, and they have not demonstrated that there is any correlation between vaccinations and autism.”

Carson’s tepid response drew immediate criticism from doctors and pediatricians across the country.

“No Ben Carson,” Baltimore pediatrician Scott Krugman wrote on Twitter. “The answer is ‘yes’ Donald Trump is wrong. Vaccines don’t cause autism. What are you talking about?”

Yet, on an issue that could prove prickly for Republicans in the general election, Carson’s comment was actually the most forceful of the night.

Trump essentially doubled down on his past statements by again suggesting that vaccines, or concentrations of them, cause autism.

“Autism has become an epidemic,” he warned. “Twenty-five years ago, 35 years ago, you look at the statistics, not even close. It has gotten totally out of control.”

Rand Paul, like Carson, a doctor, also equivocated on the issue.

“I’m all for vaccines,” he said. “But I’m also for freedom.”

The exchange, particularly Trump’s comments, drew a sharp response from autism groups.

“Despite a wealth of scientific evidence debunking any link between autism and vaccinations, tonight’s Republican primary debate featured prominent commentary from a leading candidate repeating inaccurate information suggesting a link,” the Autistic Self Advocacy Network said in a statement. “Autism is not caused by vaccines — and Autistic Americans deserve better than a political rhetoric that suggests that we would be better off dead than disabled.”

Whether or not the vaccine “debate” did any damage to Carson, Trump, Paul or the GOP among voters is still unclear. But it was a talking point from a testy night full of politicians pushing back against science and “big government.”

GOP candidates meet in second presidential debate

Republican presidential candidates, from left, former Pennsylvania Sen. Rick Santorum, former New York Gov. George Pataki, Sen. Rand Paul, R-Ky., former Arkansas Gov. Mike Huckabee, Sen. Marco Rubio, R-Fla., Sen. Ted Cruz, R-Texas, retired neurosurgeon Ben Carson, businessman Donald Trump, former Florida Gov. Jeb Bush, Wisconsin Gov. Scott Walker, businesswoman Carly Fiorina, Ohio Gov. John Kasich, and New Jersey Gov. Chris Christie take the stage during the CNN Republican presidential debate at the Ronald Reagan Presidential Library and Museum on Wednesday, Sept. 16, 2015, in Simi Valley, Calif. (AP Photo/Chris Carlson)

Technically, Carson’s answer was in line with the overwhelming scientific consensus that vaccines don’t cause autism. (The research Trump has cited has been roundly debunked; its author stripped of his medical license. In contrast, over the past 12 years, nine consecutive CDC studies have found no link between vaccines and autism.)

But even Carson’s tepid initial response quickly began to unravel.

“This was something that was spread widely 15 or 20 years ago,” he said of the supposed vaccine-autism connection, “and it has not been adequately, you know, revealed to the public what’s actually going on.”

Already fuzzy, the neurosurgeon’s statements then grew hazier and hazier, like a patient’s vision as he slips under anesthesia.

“Vaccines are very important,” Carson said, before qualifying: “Certain ones. The ones that would prevent death or crippling. There are others, there are a multitude of vaccines which probably don’t fit in that category, and there should be some discretion in those cases.”

Tapper had couched his question not only in contrast to Trump, but also in reference to the recent measles outbreak in California, where Wednesday’s debate was held.

The outbreak began in December after dozens of kids became sick with the virus at two Disney theme parks in Southern California. Nearly 200 Americans caught the disease in 2015 — 15 years after it was declared eradicated — because of the growing anti-vaccine movement, according to the CDC.

One person died in the outbreak. Does Carson think opting against the measles vaccine is okay? It’s unclear, at least from his answer Wednesday night.

When asked again if Trump should stop saying that vaccines cause autism, Carson dodged the question.

“Well, you know, I’ve just explained it to him,” he said. “He can read about it if he wants to. I think he’s an intelligent man and will make the correct decision after getting the real facts.”

But the surgeon went further, appearing to stoke anti-vaccine sentiment by adding that: “You know, a lot of this is — is — is pushed by big government.”

The Twitter reaction to Carson’s answers was split. Some felt the doctor had delivered a home run:

Some notable liberals also reveled in Carson’s comments:

But many more thought Carson struck out:

Carson’s critics also included a number of doctors:

Much of the criticism came after Carson echoed Trump’s concerns over grouping vaccines together.

“The fact of the matter is, we have extremely well-documented proof that there’s no autism associated with vaccinations. But it is true that we are probably giving way too many in too short a period of time,” Carson said. “And a lot of pediatricians now recognize that, and, I think, are cutting down on the number and the proximity in which those are done, and I think that’s appropriate.”

(Both the CDC and Institute of Medicine have said that giving children multiple vaccines at once is completely safe.)

If Carson’s equivocations raised eyebrows because he’s a doctor, Trump’s stirred outrage because … well … Trump.

After asking Carson about vaccines and autism, Tapper turned to The Donald to see if he would defend his past statements, which have repeatedly linked “massive” injections and the neurodevelopmental disorder.

“Mr. Trump, as president, you would be in charge of the Centers for Disease Control and the National Institutes of Health, both of which say you are wrong,” Tapper said. “How would you handle this as president?”

“Autism has become an epidemic,” Trump answered. “Twenty-five years ago, 35 years ago, you look at the statistics, not even close. It has gotten totally out of control.”

That claim, however, is disputed by many autism groups and studies.

“Politicians continue to talk about an autism epidemic – despite the fact that the science suggests that autism has always existed at its current rate within the general population,” the Autistic Self Advocacy Network said in response to Trump.

And on Wednesday, the Wall Street Journal reported that much of the increase in autism rates could be due to changes in the way we diagnose it, according to a series of studies.

Donald Trump was put to the test, while Carly Fiorina emerged as a big winner. (Alice Li/The Washington Post)

Under the spotlight of the Ronald Reagan Presidential Library and Museum in Simi Valley, Calif., The Donald at first appeared to back away from his controversial stance on vaccines, only to ultimately double down on it.

“I am totally in favor of vaccines,” he said. “But I want smaller doses over a longer period of time. Because you take a baby in — and I’ve seen it — and I’ve seen it, and I had my children taken care of over a long period of time, over a two or three year 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.”

Trump went so far as to recite a supposedly recent story of a toddler who had been diagnosed as autistic just a week after getting a shot and coming down with a fever.

“Just the other day, two years old, 2½ years old, a child, a 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,” he said.

Interestingly, this is not the first time Trump has told such a story. He recounted a very similar tale three years ago.

“I’ve seen people where they have a perfectly healthy child, and they go for the vaccinations and a month later the child is no longer healthy,” he told Fox & Friends in April of 2012.

“It happened to somebody that worked for me recently,” he said. “I mean, they had this beautiful child, not a problem in the world, and all of the sudden they go in and they get this monster shot. You ever see the size of it? It’s like they’re pumping in — you know, it’s terrible, the amount. And they pump this in to this little body and then all of the sudden the child is different a month later. I strongly believe that’s it.”

For several years, Trump has walked an incredibly fine — arguably nonexistent — line, arguing that vaccines are okay but that bunching them together causes autism.

And on Wednesday night he repeated his claim.

“I’m in favor of vaccines, do them over a longer period of time, same amount,” he said. “But just in — in little sections. I think — and I think you’re going to have — I think you’re going to see a big impact on autism.”

Trump’s claim — along with equivocations from doctors Carson and Paul — drew alarm from autism groups, physicians and science journalists, who saw a missed opportunity to put a dangerous myth to rest.

“While no link exists between autism and vaccines, of greater concern is the willingness of those who promote this theory to suggest that exposing children to deadly diseases would be a better outcome than an autistic child,” said the Autistic Self Advocacy Network. “Vaccinations do not cause autism – but the use of autism as a means of scaring parents from safeguarding their children from life-threatening illness demonstrates the depths of prejudice and fear that still surrounds our disability.”

For Trump in particular, Wednesday’s vaccine discussion had echoes of Michele Bachmann’s disastrous medical debate debacle four years ago. Bachmann was blasted by medical experts after saying during a GOP debate that requiring HPV vaccines was “just flat out wrong.” She then told the story of a woman whose daughter was allegedly left “mentally retarded” by an HPV injection.

“There’s a woman who came up crying to me tonight after the debate. She said her daughter was given that vaccine,” Bachmann said on Fox News. “She told me her daughter suffered mental retardation as a result. There are very dangerous consequences.”

The American Academy of Pediatrics released a statement to correct Bachmann’s “false statements.”

Whether or not scientists — and voters — punish Trump, Carson or Paul for their problematic vaccine positions will play out over the course of the election.

As for Wednesday’s debate, however, perhaps the pithiest response came from Deepa Mokshagundam, a pediatric resident at Children’s Medical Center in Dallas.

“Vaccines,” she wrote on Twitter, “are not up for debate.”

Correction: An earlier version of this article stated that no one died in the recent measles outbreak in the United States. In fact, a woman in Washington state died from pneumonia due to measles: the first measles death in the country in more than a decade.

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