More than 1,090 people across the country have been infected by the super contagious measles virus in ongoing outbreaks since January.

In New York, the virus has been spreading since September, mostly among Orthodox Jews, some of whom reject vaccines because of unfounded safety concerns. In Washington, mistrust of health officials and pharmaceutical companies drove parents in one county to opt out there — sparking another large outbreak. All together, measles clusters have sprung up in 28 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 avoidable. By 2000, thanks to the measles vaccine, the virus was declared eliminated in the US. It’s absurd that outbreaks have reappeared, yet there’s one little discussed 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 in part because of a policy failure.

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

That’s now changing. Responding to these outbreaks, lawmakers in Washington, Maine and New York passed legislation in 2019 eliminating either personal or philosophical exemptions for vaccines — or both.

But other states, like Mississippi and West Virginia, closed their loopholes years ago; their vaccine coverage rates are higher and they haven’t had to deal with any outbreaks lately.

Researchers have repeatedly shown that when you make it easier for parents to avoid vaccines, they take advantage — and vaccine exemptions rise. The reverse is also true: making it harder to opt out can drive up vaccine rates, a new JAMA analysis of California’s recent move to outlaw all but medical exemptions shows.

So if we want to prevent dangerous, costly, and needless outbreaks — like the half-dozen going on now across the country — we need to make it harder for parents to opt out.

When measles vaccine coverage drops below 95 percent, “outbreaks are inevitable”

Before we get into the wonky details of state vaccine policies, let’s pause for a refresher on why measles immunization is so essential.

It’s not an overstatement to say that measles is one of the most infectious diseases known to man. When people aren’t immunized, it’s extremely easy to catch measles. In an unvaccinated population, one person with measles can infect 12 to 18 others. That’s way higher than other viruses like Ebola, HIV, or SARS. (With Ebola, one case usually leads to two others. With HIV and SARS, one case usually leads to another four.)

In the US, before a vaccine was introduced in 1963, there were 4 million measles cases, with 48,000 hospitalizations and 500 deaths in the US every year. Measles was also a leading killer of children globally.

These days, with two doses, the measles vaccine is 97 percent effective in individuals. But for the vaccine to protect the population, including the small number of people who can’t be vaccinated, we need what’s known as herd immunity. Depending on the virus, a certain percentage of people needs to be immunized to keep a disease from spreading through populations (what’s called “herd immunity.”) Because measles spreads so easily, the percentage needed for herd immunity is really high.

“As soon as [measles] vaccination coverage drops below 95 percent,” Seth Berkley, the head of Gavi, the Vaccine Alliance, warned in a recent CNN article, “outbreaks are inevitable.” That means nearly everybody in a community who can get the vaccine needs to be accounted for to stop the virus from spreading.

The era of lax vaccine policies has to end

But state legislators have put policies in place that, in many cases, make it too easy for parents to opt out of vaccines.

It was actually measles outbreaks in the 1960s that inspired a push to have states require children 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.

Still, there’s a lot of variation across the country when it comes to immunization requirements. Even though all 50 states have legislation requiring vaccines for students entering school, 45 states allow exemptions for people with religious beliefs against immunizations, and 15 states currently grant philosophical exemptions for those opposed to vaccines because of personal or moral beliefs. (The exceptions are Mississippi, California, and West Virginia, and more recently, New York, and Maine which now 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. It’s often harder for parents to sign their kids out of school for the day than to help them avoid vaccines.

In many 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 their shots, the rates of vaccine exemptions tend to be higher. A 2018 analysis of US vaccine policies found that states allowing both religious and philosophical exemptions 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.

Every state also allows medical exemptions for people who might be harmed by a vaccine, such as those with weakened immune systems because of an illness or allergies to vaccine ingredients. And there appears to be no shortage of quack-ish health professionals who will sign off on questionable medical exemptions for people who don’t have legitimate health concerns.

What’s more, fewer than a dozen states require annual (or more frequent) recertification for medical exemptions. So for example, if a child in a K-12 school gets an exemption in kindergarten, it will follow them through to college. She’ll never be asked to renew that exemption.

So there are many ways for people to worm out of vaccines. “Putting some kind of administrative control on vaccine opt-outs is vitally important,” said Diane Peterson, the associate director for immunization projects at the Immunization Action Coalition. “It just shouldn’t be easier to get out of vaccination than it is to get vaccinated.”

California has made it tougher to opt out of vaccines — with mixed and instructive results

Some states — including New York, Maine, and Washington — have responded to the recent measles outbreaks by cracking down on vaccine avoiders.

Washington House Bill 1638 removed the personal belief exemption for the measles, mumps and rubella vaccine only.

Maine House Bill 586 got rid of both its personal and religious belief exemptions for all vaccines required for school entry.

New York Senate Bill 2994 removed the religious exemption for public school vaccine requirements.

In 2015, another measles outbreak prompted California’s former governor, Jerry Brown, to sign a bill, SB277, that abolished all nonmedical exemptions. And the California experience is instructive for other states that might want to close some of their loopholes.

According to the state health department, the number of kindergarten students in the 2017-2018 school year with all their required vaccines was 95.1 percent — a 4.7 percentage point increase over 2014-2015 and the second-highest reported vaccine rate since health authorities started tracking. A new analysis, published in JAMA, put the opt-out rate at 4.8 percent by 2017.

But hidden within that increase is some conflicting data, said Emory vaccine researcher Saad Omer. Since the law was enacted, medical exemptions have also increased, suggesting there may be an unintended effect of the crackdown on nonmedical exemptions.

Something else was going on in California, and it offset that increase in medical exemptions. To explain: In parallel with abolishing nonmedical exemptions through SB277, California launched the “Conditional Entrant Intervention Project,” in 2015. The idea was that public health professionals would work with local health departments to identify schools granting high rates of conditional entrants, and work with them to bring them down.

Omer and his colleagues found a sharp 23 percent decline in the conditional admission rate between 2014 and 2015. So even with the rise in medical exemptions, the overall vaccine exemption rate still went down thanks to the decline in conditional vaccine entry to schools.

Omer told Vox, “I’m not discounting eliminating nonmedical exemptions. It’s a reasonable option. But it may not resolve all issues.” That’s why California — which has seen 55 measles cases so far this year — is now cracking down on bogus medical exemptions, too.

There is evidence from Mississippi and West Virginia that strict vaccine laws can work — but again, interpret it with caution.

Immunization rates in Mississippi and West Virginia — the only other two US states that don’t allow non-medical exemptions — are always among the best in the nation. In the 2014-’15 school year, more than 99 percent of kindergartners in Mississippi had their MMR and diphtheria-tetanus-pertussis shots — the highest rate in the US. The rates for those vaccines were 98 percent for kindergartners in West Virginia. These figures are much higher than the national averages (85 percent for diphtheria-tetanus-pertussis and 92 percent for measles-mumps-rubella).

But these two states also have demographics that probably help vaccine coverage. Researchers have repeatedly found that parents on the higher end of the socioeconomic spectrum, including those who send their kids to private and alternative schools, tend to be the ones who opt out of vaccine. Mississippi and West Virginia are among the poorest states in the nation.

Perhaps these demographic factors contribute to the high vaccine rates in the two states, the authors of a recent analysis in Health Affairs suggested:

Among US states, Mississippi and West Virginia rank fiftieth and forty-ninth, respectively, in median income, and forty-ninth and fiftieth, respectively, in the percentage of people ages twenty-five and older who have completed a bachelor’s degree. Thus, the states may have a smaller number of residents who are likely to hold anti-vaccination views and to have the political and social capital to undertake successful efforts to influence their legislators.

That suggests that simply outlawing nonmedical exemptions may not be a panacea in states that have a high percentage of parents using their social capital to spread anti-vaccine views. And as we saw in California, a ban on nonmedical exemptions could even backfire if other vaccine loopholes are left open.

So finding ways to make it more inconvenient to opt out — by cracking down on the conditional entry to school, introducing exemptions with regular renewals — should be what policymakers work toward.

And they 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 2017-2018. Outbreaks in recent years have also been getting larger, Omer said. “That’s the canary in the coal mine for me.”