Nearly 150 public school students in Spokane, Wash., have been booted from school for lacking documentation to prove they were vaccinated.

Washington requires all public school children be vaccinated against a variety of illnesses, although parents with religious, philosophical or medical justifications are allowed to obtain a waiver. However, it appears hundreds of students were allowed to go unvaccinated without even receiving those. School officials finally took action Monday, telling hundreds of parents their kids couldn’t come back to class unless they got their shots or a state-approved waiver.

The move was inspired by a recent 2015 measles outbreak, which has affected 147 people in seven states, including two in Washington.

“[The students will] stay out of school until they show compliance,” district spokesman Kevin Morrison said. He also remarked that he believed this was the first such crackdown in the state. “Given the increased awareness that the world has of the possibility of pandemic, I think heightened awareness is certainly not a bad thing.”

In addition to being the first in Washington, Spokane’s actions also constitute one of the most decisive actions anywhere in the country by a school district not immediately coping with a sudden disease outbreak.

This first crackdown may be only the beginning. According to local KREM news, over 900 students in the district (about 3 percent of the total) have been found to have imperfect immunization records, so even more kids may be sent home to get their shots.

Keeping accurate vaccination records is harder than it sounds, as many parents children in Spokane’s schools were born in other states or overseas, and many have parents who do not speak English.

The refusal of many parents to vaccinate their children has surged back into the public consciousness in recent months, as America is hit by new outbreaks of diseases like measles that until recently had been eradicated in the U.S. Some parents believe that vaccines are unnatural, while others fear they contribute to conditions like autism despite a lack of scientific evidence that is the case. New outbreaks have caused many politicians and health experts to question laws that allow parents to receive vaccine exemptions for non-medical reasons.