The science is clear on vaccines’ safety and efficacy. But the presidential candidates’ thoughts on vaccines? Well, they’re a mixed bag.

Over the past week, Green Party candidate Jill Stein — who’s a medical doctor by trade — entered the heated political debate over vaccines by suggesting there are still "real questions" about the medical devices that scientists say are generally effective and safe. She later added that she supports vaccines, but has questions about how they’re regulated.

The small blow-up put attention on yet another bizarre element of the 2016 election: Out of the four big presidential candidates, only Hillary Clinton seems to be fully pro-vaccine, meaning she’s the only one aligned with the scientific consensus on this issue. Republican Donald Trump is a straight-up anti-vaxxer, and the other two candidates — Stein and Libertarian Gary Johnson — have mixed views on the issue.

Before we get to that, though, let’s go back to what got Stein in trouble in the first place.

Jill Stein has repeatedly raised questions about vaccines’ safety and efficacy

Stein told the Washington Post:

"I think there’s no question that vaccines have been absolutely critical in ridding us of the scourge of many diseases — smallpox, polio, etc. So vaccines are an invaluable medication," Stein said. "Like any medication, they also should be — what shall we say? — approved by a regulatory board that people can trust. And I think right now, that is the problem. That people do not trust a Food and Drug Administration, or even the CDC for that matter, where corporate influence and the pharmaceutical industry has a lot of influence." … "As a medical doctor, there was a time where I looked very closely at those issues, and not all those issues were completely resolved," Stein said. "There were concerns among physicians about what the vaccination schedule meant, the toxic substances like mercury which used to be rampant in vaccines. There were real questions that needed to be addressed. I think some of them at least have been addressed. I don’t know if all of them have been addressed."

She later added on Twitter:

I'm not aware of evidence linking autism with vaccines. Let's do more to support autistic people & their families. https://t.co/eISgfxQ5vm — Dr. Jill Stein (@DrJillStein) July 31, 2016 As a medical doctor of course I support vaccinations. I have a problem with the FDA being controlled by drug companies. — Dr. Jill Stein (@DrJillStein) July 29, 2016

So in the Washington Post interview, Stein said there are "concerns among physicians about what the vaccination schedule meant, the toxic substances like mercury which used to be rampant in vaccines. There were real questions that needed to be addressed." This seemed like a dog whistle and pandering to anti-vaxxers, who say vaccines are unsafe and cause autism, despite the overwhelming scientific evidence that shows otherwise.

And while Stein’s follow-up tweets say she supports vaccinations and isn’t aware of any evidence that they cause autism, they don’t contradict her claims that there are "real questions" about vaccines and that doctors have "concerns" about vaccines — two things that make it seem like the science on vaccines’ safety and efficacy is shakier and more mixed than it really is.

Stein has a history of pandering to anti-vaxxers. In a Reddit Ask Me Anything session in May, which was recently resurfaced by Jezebel, Stein said:

According to the most recent review of vaccination policies across the globe, mandatory vaccination that doesn’t allow for medical exemptions is practically unheard of. In most countries, people trust their regulatory agencies and have very high rates of vaccination through voluntary programs. In the US, however, regulatory agencies are routinely packed with corporate lobbyists and CEOs. So the foxes are guarding the chicken coop as usual in the US. So who wouldn’t be skeptical? I think dropping vaccinations rates that can and must be fixed in order to get at the vaccination issue: the widespread distrust of the medical-indsutrial [sic] complex.

Similar to the Washington Post interview, Stein’s Reddit post goes on to say that vaccines have "in general have made a huge contribution to public health." So while Stein doesn’t seem to support mandatory vaccination and doesn’t like how they’re regulated, she doesn’t personally appear to view them as dangerous.

So Stein is walking a fine line: She seems to know that vaccines are safe and effective, but she’s fostering just enough skepticism about her stance to pander to anti-vaxxers.

Perhaps the clearest example of this, as Gizmodo reported, is how Stein changed the wording of her tweet in response to a question about whether she believes vaccines can cause autism. She went from tweeting that "There’s no evidence that autism is caused by vaccines" to "I’m not aware of evidence linking autism with vaccines." The subtle tweak casts way more doubt than is necessary, considering there really is no evidence that vaccines cause autism.

Stein also suggested that federal regulators who oversee vaccines are controlled by the medical industry. But as Dave Weigel reported for the Washington Post, "most members of the Vaccines and Related Biological Products Advisory Committee work at academic or medical institutions, not drug companies." So her claim seems to be wrong, based on the actual regulatory body for vaccines.

Still, for Stein, there are some political incentives to take this kind of stance: Part of her Green Party base is made up of science-skeptical liberals who think that vaccines are potentially dangerous and linked to autism, and some just believe that federal regulators are under control of the medical industry, regardless of their vaccine beliefs. To keep the Green Party coalition together, Stein likely feels that she needs these people under her banner.

Among the four big candidates, only Hillary Clinton is fully pro-vaccine, and Donald Trump is full anti-vaxxer

Among the four big presidential candidates, Stein isn’t alone. Here’s how the other candidates break down:

Democrat Hillary Clinton has said that vaccines work and that all kids should be vaccinated. (Still, Clinton wasn’t always so sure: In a 2007/2008 questionnaire for an autism advocacy group reported by Mother Jones, Clinton wrote, "I am committed to make investments to find the causes of autism, including possible environmental causes like vaccines. … We don't know what, if any, kind of link there is between vaccines and autism — but we should find out.")

Republican Donald Trump is a total anti-vaxxer, repeatedly claiming over the years that vaccines hurt kids and cause autism. Needless to say, he opposes mandatory vaccination.

Libertarian Gary Johnson’s personal views on vaccines aren’t totally clear, but he voiced opposition to mandatory vaccination in the past.

So Clinton is the only major candidate running for president who has a fully pro-vaccine (and pro-science) position. Trump is clearly the most anti-science, anti-vaccine candidate. And Johnson and Stein seemingly oppose the scientific consensus on mandatory vaccination. It’s a very odd mix, considering the weight of the evidence.

Anti-vaccine views contradict the medical consensus

The issue with the anti-vaccine position — and Stein, Trump, and Johnson’s apparent opposition to mandatory vaccination in particular — is, well, science.

The medical consensus on vaccines is fairly clear: They are generally safe and effective, and all eligible kids should be vaccinated. A 2014 survey from the Pew Research Center found that 86 percent of scientists with the American Association for the Advancement of Science think childhood vaccines should be mandatory. And the American Academy of Pediatrics, Centers for Disease Control and Prevention, National Institutes of Health, National Academy of Sciences, and World Health Organization encourage vaccination and deem it safe.

The medical field’s thinking here is simple: For vaccines to really work, nearly everyone has to take them. The reason for this is what’s known as "herd immunity," which refers to the idea that a lot of people need to get a given vaccine, whether it's for the flu or measles, to stop a disease from spreading. Vaccinated people essentially act as barriers to outbreaks, since diseases can't pass through them and infect others.

This barrier helps protect some of the most vulnerable populations: infants under 12 months of age, who can't get vaccinated and are more susceptible to infection; the elderly, who have a higher risk of death if they contract vaccine-treatable illnesses; and people with compromised immune systems, who can't get vaccines and are more likely to die from the diseases they protect against.

So if enough people pass on vaccines, it can put these vulnerable populations — and others who skip on vaccines — in serious danger, creating the risk of an unnecessary epidemic.

But in yet another bizarre aspect of the 2016 election, most of the big presidential candidates — with the exception of Clinton — seem to oppose the scientific consensus on vaccines.

Watch: The weird and fascinating history of the anti-vaxxer movement