An ongoing outbreak of measles — one of the most infectious diseases known to man — in Washington state has prompted the governor to declare a public health emergency.

As of Monday, there were 35 cases in Washington’s Clark County, which borders Portland, Oregon. Most of the cases involved children between 1 and 10 years old who had not been vaccinated. There’s also one adult case in King County, whose largest city is Seattle.

“The measles virus is a highly contagious infectious disease that can be fatal in small children,” Washington’s governor, Jay Inslee, said in his emergency declaration on Friday. The fact that it’s spreading in Washington, since the outbreak was identified on January 18, creates an “extreme public health risk that may quickly spread to other counties.”

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The reason for the emergency is simple: Measles is extremely contagious. A person with measles can cough in a room and leave, and — if you are unvaccinated — hours later, you could catch the virus from the droplets in the air that they left behind. No other virus can do that.

In the Clark County outbreak, people with the virus had visited public places including health care facilities, schools, and churches, as well as Ikea and Dollar Tree — potentially spreading measles to others.

That leads us to another fact that makes the outbreak in the Northwest particularly troubling: There are likely to be more at-risk children in those public places than in most of the rest of the country. That’s because Oregon and Washington are more permissive than other states when it comes to allowing parents to opt out of vaccines — and many parents have been taking advantage of that.

In Clark County, 7.9 percent of children had gotten exemptions from vaccines for entry to kindergarten in the 2017-’18 school year, according to the Washington Post. And across the river in Oregon, the rate of vaccine exemptions has sharply increased lately, from 5.8 percent in 2015 to 7.5 percent in 2018. That’s much higher than the national average, which suggests 2 percent of children go unvaccinated for nonmedical reasons.

These opt-outs have made Oregon and Washington more susceptible to entirely preventable outbreaks, said Peter Hotez, a Baylor College of Medicine infectious diseases researcher.

“[They] are now a major anti-vaccine hotspot due to nonmedical vaccine exemptions that have nothing to do with religion,” he said. While Oregon hasn’t seen any cases yet, the geographic proximity of the Washington outbreak is worrisome, said Hotez. And “because this is such a large unvaccinated cohort,” he added, “this epidemic could last a while.”

States with tougher vaccine laws have higher rates of vaccination

Measles can be easily prevented with the measles-mumps-rubella vaccine, which children in the US are supposed to get before entering kindergarten. Of those who get the two doses of the vaccine, almost none will ever contract measles, even if they’re exposed.

Across the country, 91 percent of young children got the MMR vaccine, according to the CDC’s latest data in 2017. That’s nearly enough for what’s known as “herd immunity”: in order for any vaccine to be effective, you need to have a certain (high) percentage of people in a population immunized. Herd immunity keeps diseases from spreading through populations very easily, and keeps vulnerable groups who can’t be vaccinated (such as very young babies or people with allergies to vaccines) protected.

But even though people have a social responsibility to vaccinate their kids — and some countries have been experimenting with punitive measures to censure parents who out out — many US states are more permissive of vaccine refusers.

All 50 states have legislation requiring vaccines for students — but almost every state allows exemptions for people with religious beliefs against immunizations, and 17 states 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 no philosophical or religious exemptions.)

“States expect that in order to access public resources, like schools, camps, or child care centers, individuals must give up some autonomy to make sure everyone in the community is safe,” University of Colorado Denver sociologist Jennifer Reich, who studies the anti-vaccine movement, told Vox previously.

Perhaps unsurprisingly, the parts of the country that make it easier for people to opt out of their shots tend to have higher rates of ... people opting out of vaccines. So a lax regulatory environment can create space for more parents to refuse vaccines. And that’s what we’re seeing play out in the Northwest right now.

Oregon and Washington are among the 17 states in the US that allow philosophical vaccine exemptions for people who want to opt out because of moral or personal beliefs. In 12 of these states, the number of these exemptions has risen since 2009.

In 2015, Oregon state lawmakers made it a bit more difficult for parents to exempt their kids from vaccines — requiring moms and dads to watch an online education vaccine program or talk to a doctor or nurse to get a “vaccine education certificate.” But that hasn’t deterred parents, according to a recent state analysis. After an initial dip in the nonmedical exemption rate, it’s steadily increased over the past few years.

“Measles was eliminated from the US in 2000, but it’s been allowed to return,” said Hotez, in part because of “ignorant and cowardly state legislatures, and a failure by governments to mount a pro-vaccine advocacy campaign.”