Students attending hundreds of schools in the state of Washington could face an increased risk of contracting some preventable diseases because immunization completion rates aren't high enough to protect them in the event of an outbreak.

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"What that means is there's an opportunity for us to prevent more illnesses, missed days of school, doctors visits and hospitalizations by maximizing the number of students who are in compliance with school immunizations," said Jeff Duchin, the public health officer for Seattle and King County.

Because nearly all students have to be vaccinated to effectively prevent the spread of most diseases, the state set a goal for all Washington schools to have 95 percent of their kindergarten students complete all immunizations. But statewide, most schools have consistently failed to meet that target each year, according to a KING 5 review of annual vaccination data reported to the Washington Department of Health between 1988 and 2017.

It's the percentage of people who must be vaccinated for a disease in order to protect the people who aren't vaccinated from getting sick. Every disease has it's own herd immunity threshold. To prevent the spread of measles, for example, 83 to 94 percent of a community must be vaccinated, according to the National Institute of Allergy and Infectious Disease.

Each year, the majority off Washington kindergarten students get the required vaccines, but public health officials say they become concerned when they notice large numbers of un-immunized students clustered together in school settings. The science is clear that the choice parents make to vaccinate or not to vaccinate their children can affect the entire community – all because of a sheltering effect called "herd immunity."

SEATTLE – Students attending hundreds of schools in the state of Washington could face an increased risk of contracting some preventable diseases because immunization completion rates aren't high enough to protect them in the event of an outbreak, a KING 5 analysis of statewide vaccination rates found.

Chapter two : Which School Districts Fell Short?

Kindergarten vaccination rates are enforced at the school district level. While the state Department of Health tracks school-reported immunization data, officials say their data alone cannot pinpoint how accurate the risk is. Still, the kindergarten rates are important because they provide public health officials with the first insight into the possibility of an outbreak in a community.

"We know that pockets of un-immunized children – that could be in a classroom or that could be in a neighborhood – are really the highest risk for spread to others in that area," said Duchin, the King County public health officer. "It could be families, community, neighbors, even co-workers of parents when parents become infected."

Washington is no stranger to outbreaks. Small and localized outbreaks of pertussis happen every year, and larger outbreaks of the disease occur every three to five years, according to the Washington Department of Health. In 2012 and 2015, there were larger-than-average outbreaks of whooping cough across the state. The disease is highly contagious, and medical professionals say it can be deadly – especially for children who are too young to get the vaccine.

More than 700 schools – nearly half of the schools that reported kindergarten immunization last year –did not reach the 92 percent minimum herd immunity level for pertussis. That includes schools with more than 100 kindergarten students, like Bellevue's Children's Academy and Skyline Elementary School in the Lake Stevens School District, which both had a 72 percent completion rate. Sunnycrest Elementary School in the Federal Way district also fell short, with 68 percent of its kindergarten students vaccinated for pertussis, according to the DOH data.

Washington saw more than 800 cases of mumps between October 2016 and May 2017, affecting 500 people 18 years and older. For the mumps vaccine to be effective, epidemiologists say at least 75 percent of the people in a setting must be immunized for it. Most Washington schools either met that minimum threshold last school year or exceeded it, according to the KING 5 analysis. But 175 Washington schools had completion rates below the herd immunity level.

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. In August, health officials confirmed a measles case in King County. The only confirmed death due to measles in the U.S. since 2003 occurred in Clallam County in 2015.

"Immunization rates are just something we can't take our eyes off of. It's an ongoing body of work to promote the benefits of full vaccination," said Kay Knox, who runs Within Reach, a nonprofit Washington-based organization that educates the community about the value of immunization. "There's a global risk if we don't have community immunity.

Click the arrows on the graphic below to find out which schools fall short of reaching herd immunity levels for specific diseases, like whooping cough and measles.

In the 2017-2018 school year, 1,177 Washington schools that reported kindergarten vaccination data to the Department of Health had immunization rates below the state's 95 percent completion target.

"We would love to see all schools have 95 percent or higher completion rates, but that's not always possible in smaller schools – like when a school has only four kids in it," said Danielle Koenig, Immunization Health Promotions Supervisor at the Washington Department of Heath.

While the majority of Washington schools do not meet the state target, Koenig says that doesn't mean that students in every one of those schools are at risk – especially since some schools that fall short are close to hitting the mark.