Measles is spreading quickly in several parts of the country, with more than 700 cases reported in 22 states. That’s already more cases than in any other year since 1994 — and it’s only April.

In New York, the state with the largest number of measles cases, the virus has been on the move since last September, mostly among Orthodox Jews, some of whom reject vaccines because of unfounded safety concerns. That outbreak sparked another in Detroit. In Washington state, where another big outbreak just ended, mistrust of health officials and pharmaceutical companies drove parents to opt out. There are also measles clusters in New Jersey, Michigan, Maryland, and California, among other states.

These outbreaks will cost states and the federal government millions of dollars to contain. They’ll distract from other important public health programs. Most importantly, they’ll put people who can’t be immunized — newborn babies, kids with vaccine allergies — at risk.

But here’s the most frustrating part: This is all entirely preventable. By 2000, thanks to the highly effective measles vaccine, the virus was declared eliminated in the US. It’s absurd that outbreaks have reappeared.

Yet there’s a major reason why: Too many states make it way too easy for parents to avoid vaccines on behalf of their kids. In other words, measles is making a comeback because of a policy failure.

Most of the people with measles right now weren’t immunized from the virus. And many live in places that permit a variety of nonmedical (religious or philosophical) exemptions from vaccines.

The outbreaks here have mostly started like this: An un-immunized American picks up the virus while traveling in a country where measles is spreading more broadly and then brings it back to their undervaccinated, tight-knit community in the US. (The top three countries where measles cases in the US originated were Ukraine, Israel, and the Philippines.

States give parents too many ways to avoid vaccines

To understand why it can be easy to opt out of vaccines, you need to understand our national system of vaccine requirements. It’s best understood as an exercise in federalism: There’s a ton of variation across the country when it comes to individual immunization mandates.

It was actually measles outbreaks in the 1960s that inspired a push to have states require children to get inoculated before starting kindergarten. By the 1980s, all states had mandatory immunization laws in place. The idea behind these laws was simple: Near-universal vaccinations sustain herd immunity.

But even though every state has legislation requiring vaccines for students entering school, almost all of them allow exemptions for people with religious beliefs against immunizations, and 17 states currently grant philosophical exemptions for those opposed to vaccines because of personal or moral beliefs. (The exceptions are Mississippi, California, and West Virginia, which have the strictest vaccine laws in the nation, allowing only medical exemptions.)

In these places, opting out can mean simply listening to a doctor or health official explain the benefits of vaccination or getting a signed statement about your religious beliefs notarized. In 45 states, even without an exemption, kids can be granted “conditional entrance” to school on the promise that they will be vaccinated, but schools don’t always bother to follow up.

We have plenty of evidence, spanning more than a decade, to show that when you make it easier for parents to opt out of shots, the rates of vaccine exemptions tend to be higher. The most recent 2018 analysis of US vaccine policies found that states allowing both religious and philosophical exemptions — as 17 states currently do — were associated with a 2.3 percent decrease in measles-mumps-rubella vaccine rates and a 1.5 percent increase in both total exemptions and nonmedical exemptions.

And many of the states with more permissive vaccine exemption policies — such as New York, Washington, and Michigan — are where we’re seeing outbreaks today. California, home to one of the fastest-growing outbreaks right now, is an exception. In 2015, the state abolished nonmedical exemptions, but the vaccine mandate applied to school-age kids, and the outbreaks there have mostly affected adults. So even closing vaccine loopholes after generations of permissiveness won’t necessarily capture everyone who’s unvaccinated.

Travelers have been bringing measles back to tight-knit communities, where it’s spreading

Another thing the measles outbreaks have in common: 88 percent of all cases have involved outbreaks in what the Centers for Disease Control and Prevention calls “close-knit communities,” or people of a similar background who share values and beliefs and interact often.

This phenomenon isn’t specific to one religion or cultural background. This year, measles has spread among Orthodox Jews in New York and the Slavic community in Washington. In years past, we’ve seen explosive outbreaks among the Amish in Ohio and Somali Americans in Minnesota.

Tight-knit communities have become an urgent focus of health departments across the country, Nancy Messonnier, the director of the CDC’s National Center for Immunization and Respiratory Diseases, told Vox earlier this year. When measles strikes, outbreaks in these groups tend to be “explosive” and more difficult to control.

While the reasons for vaccine skepticism may be different in each of these communities, the groups themselves have a lot in common. They’re cohesive and conservative. They appear to trust each other more than outsiders. They also speak the same languages and read or watch the same news. This means it’s easy for anti-vaccine rhetoric to spread — and viruses too. “We think these communities are more alike,” Messonnier added, and their insularity helps “outbreaks escalate.”

So more than Facebook, real-life social networks seem to turbocharge the spread of anti-vaccine views, and along with them viruses like measles. But again, we’d have little issue in these communities if it wasn’t so easy to opt out of vaccines.

So what can we do to stop it?

In the current outbreak, states and cities have been taking extraordinary measures to get people vaccinated — from fining those who opt out to mandating vaccines where the virus is spreading and closing down grade schools or quarantining students. Washington’s state Senate also passed a bill to eliminate personal and philosophical exemptions for the measles-mumps-rubella vaccine. (It still needs to be signed into law.)

But it’s not clear these crackdowns amid ongoing outbreaks will help. They’re reactive and heavy-headed — and they might backfire, causing the skeptics of vaccines to dig in. Especially when it comes to tight-knit communities, public health officials need to build trust over years, and stick around long after outbreaks are over, to get communities and community leaders on their side.

States also need to find ways to simply make it more inconvenient to opt out — by cracking down on things like the conditional entry to school, or introducing exemptions with regular renewals. These measures are subtler than mandates or fines and may be more effective.

States should move fast. The percentage of people seeking nonmedical exemptions — while still small — has also been creeping upward, from 1.1 percent in 2009-2010 to 2.2 percent by 2017-’18. Outbreaks in recent years have also been getting larger, Emory vaccine researcher Saad Omer told Vox earlier this year. “That’s the canary in the coal mine for me.”