If there was ever a moment to fear the anti-vaxxers could win, this is it.

Consider the following:

The developments are worrying enough that two top vaccine researchers published op-eds this year, warning about the risk of more outbreaks of vaccine-preventable diseases like measles and mumps.

“Even a modest decrease in [vaccine coverage] rates could be enough to cause future outbreaks,” wrote Peter Hotez, a pediatrician at Baylor College of Medicine, in the New York Times. “I’m worried that our nation’s health will soon be threatened because we have not stood up to the pseudoscience and fake conspiracy claims of this movement.”

Hotez and Saad Omer, a leading vaccine researcher at Emory University who wrote the other op-ed, are right to be concerned.

Vaccine refusals are rising in many parts of the country because states have made it easier for parents to opt out

Let’s be clear about something up front: Most American children still get their shots. More than 90 percent of kids receive vaccines for polio, measles, mumps, rubella, hepatitis B, and chickenpox, though the coverage rates are slightly lower for other routine vaccines. Most American parents also say they support school-based vaccine requirements.

States started to mandate that school children get inoculated against diseases because we need vaccination rates to remain high to sustain what’s called “herd immunity.” For any vaccine to be effective and prevent outbreaks, a certain (high) percentage of people in a population need to be immunized. This keeps diseases from spreading easily, and keeps vulnerable groups that can't be vaccinated (such as very young babies or people with allergies to vaccines) protected.

And yet, since vaccination was invented more than 200 years ago, anti-vaxxers have been organizing. The seeds of the modern anti-vaccine movement can be traced back to bogus 1990s research by Andrew Wakefield, a discredited physician-researcher who linked the measles-mumps-rubella vaccine to autism. Since then, a small but vociferous community of activists — abetted by doctors who enable them — have been spreading unfounded fears among parents about vaccine safety.

As David Gorski, a surgeon who has tracked the movement, wrote in a recent blog post, “Hard-core antivaxers are a minority. They are and remain cranks. … Unfortunately, they have an outsized influence on reasonable parents who just don’t have the scientific background to recognize their misinformation and pseudoscience for what they are, contributing to vaccine hesitancy.”

So how great is their influence? And is it growing stronger? In the past couple of years, there haven’t been any new peer-reviewed studies on national trends in vaccine refusal. But the latest evidence we have from individual states, in combination with older studies on vaccine coverage rates and recent surveys of doctors, suggest there’s a growing problem in several parts of the country.

The main way to measure how many kids aren’t getting vaccinated by choice is finding out which proportion of kids get exempted from school vaccine requirements for nonmedical — that is, personal belief or religious — reasons. Since immunization laws are state-based, there’s variation across the country when it comes to the requirements. As of August 2016, all 50 states have legislation requiring vaccines for students — but almost every state allows exemptions for people with religious beliefs against immunizations, and 18 states grant philosophical exemptions for those opposed to vaccines because of personal or moral beliefs (with the exception of Mississippi, California, and West Virginia, which have the strictest vaccine laws in the nation).

Leah Samuel of Stat News crunched the latest Centers for Disease Control and Prevention data on nonmedical exemptions from 2009 to 2016 (her numbers, however, weren’t peer-reviewed, like some of the other studies I’ll describe next). She found that the volume of people seeking exemptions was greater in 2016 than at any other point since 2009 in 11 states: Connecticut, Florida, Iowa, Kentucky, Maryland, New York, North Carolina, North Dakota, Ohio, Oklahoma, and Virginia. She also found the national average for nonmedical exemptions was down in 2016 from a 2009 spike — a fact she attributes to California and Vermont’s 2015 cancellation of their personal belief exemptions.

Some of the best peer-reviewed evidence for an increase in the number of people refusing or delaying vaccines comes from Omer at Emory.

In one 2009 New England Journal of Medicine paper, Omer looked at the state-level rates of nonmedical exemptions. He finds that between 1991 and 2004, those rates increased from less than 0.98 percent to about 1.5 percent. Again, this uptick was not spread evenly across the US, and even varied within states.

Generally, though, states that allowed only religious exemptions had a steady opt-out rate of about 1 percent during the period (1991 to 2004). But in states that were more lax — allowing philosophical or personal belief exemptions as well as religious and medical exemptions — the mean exemption rate increased from 0.99 to 2.54 percent.

In a 2012 follow-up to that paper, also published in NEJM, Omer found — once again — that “non-medical exemptions have continued to increase, and the rate of increase has accelerated.”

Doctors are also reporting that they’re meeting more and more parents who are skeptical of vaccines. The American Academy of Pediatrics periodically surveys its members and identified a rise in pediatricians reporting that they had patients who refused a vaccine — from 75 percent in 2006 to 87 percent in 2013.

As for state-level analyses, Baylor’s Peter Hotez looked at the rate of nonmedical exemptions over the past 13 years in his home state of Texas. He found that in 2016, there were almost 45,000 children who refused vaccines — about double the number of exemptions in 2010 and a 19-fold increase compared with 2003:

Texas is one of those lax states that allow parents to get both religious and philosophical vaccine exemptions. And other studies have shown that people who live in places that make it easy to opt out of vaccines tend to have higher rates of exemptions. In one paper, states that allowed parents to refuse vaccines for philosophical or personal reasons had exemption rates that were 2.54 times as high as states that only permitted religious exemptions. Another older study, looking at data between 1991 and 2004, found an increase in exemption rates only in states that allowed philosophical exemptions.

The “vaccine hesitant” are more common than outright vaccine deniers

There’s also evidence of another growing group of vaccine skeptics: More common than outright vaccine deniers are parents who might be better described as “vaccine delayers.” They generally agree that vaccination is a public health benefit, and they don’t identify as "anti-vaxxers." But they’re hesitant and skeptical about some areas of vaccine science, and they work with doctors to create their own vaccine schedules, choosing which vaccines to give their kids and which to skip.

“There are numerically few parents who exercise exemptions entirely against all vaccines,” said sociologist Jennifer Reich, who studies the anti-vaccine movement, in an email. “But there is evidence that as many as 20 to 25 percent of American parents feel uneasy about the number of vaccines given, feel uncertain they are entirely necessary, and wonder if they are safe.”

There are real public health consequences to these exemptions, too. As I reported last year, researchers have found that rates of whooping cough and measles are higher among people who are intentionally unvaccinated. Unvaccinated individuals were often "patient zeroes" in state outbreaks, meaning they sparked disease clusters by creating pockets of susceptibility that caused others to fall ill.

Vaccine refusers are a hugely diverse group — but they do share some common characteristics

So who exactly are the vaccine refusers putting public health at risk?

A politically and geographically diverse bunch, it turns out. “In Oregon, it’s an organic movement appealing to natural things and that we shouldn’t do unnatural things to kids’ bodies,” said Hotez. “In Texas, it seems it’s more about choice and civil liberties. ‘First they take our guns away, and now they’re trying to make us vaccinate our kids.’” There are clusters of people who refuse vaccines for religious reasons across the country, too.

Overall, though, they do have some things in common. Most research shows that parents who reject some or all vaccines are more likely to be white, college-educated, and married, with higher family income. They also do so for personal reasons — which tend to be concerns about vaccine safety.

That squares with Reich’s findings. She interviewed a group of parents over 10 years to learn about their vaccine views, and found “the university-educated ubermoms who favor organic food and have a tendency to avoid gluten and dairy products” were among the vaccine-hesitant, according to Times Higher Education.

In an email, she told Vox that she thinks “these factors allow parents to feel they can manage illness and health without vaccines.” In other words, families with more resources may believe they can deal with serious consequences that arise if their kids become sick. “They can afford to miss work, access hospitals, and feel best able to manage risks,” she added.

That’s just parents doing what parents do, Reich said: trying to make the best decisions for their kids. Unfortunately, their decisions aren’t rooted in science, and, she added, “Their efforts to protect their own children by avoiding vaccines may actually endanger [other people] in the community, including some who are most vulnerable to the worst outcomes of infection.”

Trump may embolden the anti-vaxxers — but states have the power to push back

Every health researcher I talked to for this piece worried that President Trump’s seeming willingness to court anti-vaxxers like Robert F. Kennedy Jr. is emboldening the movement.

Infectious diseases doctor Paul Offit has linked the growing vaccine refusal to “[this] age where science denialism is institutionalized” — noting that several of Trump’s Cabinet picks hold views that are antithetical to science. “It’s just a world turned upside down.”

“[Vaccine denialism] could get worse under Trump,” Omer said, “not only because of his rhetoric but because of the cuts to the public health funding,” referring to the fact that money for public health measures like immunizations is on the chopping block under the current Republican Congress.

The vaccine researchers also said that America’s public health community isn’t addressing the problem.

“The CDC, NIH, HHS — they don’t really want to talk about [vaccine refusers],” Hotez said. “They think if we don’t make a big deal of this problem, it will go away.”

But both Hotez and Omer would like to see more outreach by public health officials into anti-vaccine communities, and see national leaders at least acknowledge the problem and defend the lifesaving value of vaccines. “Obama, Bush, Clinton, even with all the craziness around the Wakefield [vaccine autism] paper, didn’t really have much of a voice on [this issue],” said Hotez. “It’s frustrating for me.”

This leaves ample space for the states to be more active on the issue. Again, we know that when state governments make it easier for parents to opt out of vaccines with different exemption provisions, they’re more likely to exercise that right. And lately, states have been doing a good job of fighting back against anti-vaccine forces.

This JAMA paper is instructive. The researchers found that between 2009 and 2012, states considered 36 bills related to vaccine exemptions: 31 wanted to expand them, making it easier to opt out of vaccines, while only five wanted to make vaccine exemptions more difficult to obtain. None of the 31 anti-vaccine bills passed, while three of the five bills clamping down on vaccine deniers made it, which suggests that while there was more activity from the anti-vaccine side, public health won out in state legislatures.

So states’ role in ensuring communities are immunized is more important than ever. They’ll need to be strong and organized under a president who is unlikely to be a champion of vaccines and a Congress poised to cut public health funding.