King County had the third-highest number of parents claiming exemptions from vaccine requirements for their children last year, according to a new study. Experts warn that this may leave residents of Washington’s most populous county vulnerable to outbreaks of deadly disease.

Washington is one of 18 states that currently allows parents to refuse to vaccinate their children for reasons other than religious practice. Non-medical exemptions, or NMEs, have been granted to Washington residents since mandated immunizations began in the state in 1979. Statewide, the vaccination rate for measles, mumps, and rubella (known collectively as the MMR vaccine) is just over 90%. Experts say a 90–95% rate is required for “herd immunity,” the protection of those who are unable to be vaccinated.

Despite more rigorous requirements enacted in the state in 2011, the rate of kindergarten vaccination has remained steady over the past five years, after an initial increase. Because immunization is enforced at the school district level, kindergarten vaccination rates provide the first insights into the possibility of an outbreak in a community.

Washington remains the state with the third-lowest vaccination rate against MMR.

In a study published this week in PLoS Medicine, scientists from Baylor College of Medicine examined state-by-state and county-by-county data of the 18 states that allow NMEs. By providing the first such comparison of vaccination rates across the nation, they revealed an increase in NMEs in the Pacific Northwest over the past decade.

According to Dr. Peter Hotez, the dean of the National School of Tropical Medicine at Baylor and one of the authors of the study, national health agencies such as the CDC pay little attention to the anti-vaccine movement because nationally, the levels of exemption are low. However, he continues, “The real problem is at local levels. There are certain counties and school districts in the United States where there is a large percentage of kids not getting vaccinated.”

This may leave Seattle, and other such cities, vulnerable to outbreaks of childhood diseases once eradicated from the country. Washington state has already seen major outbreaks of mumps in 2017 and pertussis, or whooping cough, in 2015. The Centers for Disease Control recommend vaccinations against pertussis begin at two months and MMR at one year of age.

In the 2017–18 school year, six school districts in King County had kindergarten vaccination rates for MMR and/or pertussis below 90%, including Seattle Public Schools.

“One of our key directives is to make sure that students who attend our schools are protected by vaccines,” says Marie DeBell, the manager of health services for Seattle Public Schools. “We work really, really hard throughout the school year to bring our students into compliance.”

The school district currently offers free vaccinations on-site with parental permission and its nurses are trained in talking to vaccine-hesitant parents by the Immunization Action Coalition. Dr. Jeffrey Duchin, the health officer for public health in Seattle and King County, praises the state’s universal vaccination program but says there’s clearly still work to be done to address the remaining gap in coverage.

“It’s a complicated issue,” Duchin says. “There’s not a single reason for parents to choose to exempt their children and there’s not really a good one-size-fits-all solution.”

While doctors can’t say with certainty what causes any given outbreak, “there is good data to suggest that unvaccinated children are at risk for serious infections and can promote the spread of vaccine-preventable diseases in their community,” says Doug Opel, associate professor of pediatrics at the University of Washington School of Medicine and a general pediatrician at Seattle Children’s.

Measles, eliminated from the U.S. in the early 2000s, has been cropping up across the country in recent years. High-profile outbreaks occurred in Kansas City this year and Minneapolis in 2017. Both Missouri and Minnesota have higher rates of vaccination than Washington. The only confirmed death due to measles in the U.S. since 2003 occurred in Clallam County in 2015; typically, 1–2 deaths will occur per 1,000 cases.

Why should we be concerned about measles?

“It’s a killer disease,” warns Hotez. “Back in the early 1980s, measles was the single leading killer of children in the world.”

WATCH: CDC: 3 Out of 4 Children Killed by Flu in U.S. Were Not Vaccinated