School districts are working to ensure thousands of Washington students follow a new law that prohibits a personal exemption for the MMR vaccine.

SEATTLE — As districts headed back to school, thousands of students across the state who claim personal vaccine exemptions must receive the measles, mumps, and rubella vaccine, or they could face being excluded from school.

A new law that went into effect in July removes the personal and philosophical exemption for the MMR vaccine and requires those students either get vaccinated for those viruses or claim a religious or medical exemption for the MMR vaccine in order to attend public schools and daycares.

In the 2018-19 school year, 3.7% of exempted students, or 41,871 kids, claimed a personal exemption, according to data from the Washington State Department of Health.

In Seattle, the district said it sent letters in June to families of 300 students who claimed a personal exemption and informed them of the changes. If those students still aren’t in compliance by early October, Seattle Public Schools spokesperson Tim Robinson said the district will send “pre-exclusion” letters giving students 30 days to show they are either vaccinated or in the process of getting vaccinated. After that, non-compliant students won’t be able to enter district buildings.

“This is obviously something that we do not want to happen, which is why we are working so hard now on this issue,” Robinson wrote in an email.

There are also approximately 7,000 students in Seattle Public Schools that do not have up-to-date paperwork, and Robinson said they were sent letters in August informing them to update their information.

Other districts across the state are going through a similar process.

Lake Washington School District, which is the second-largest school district in western Washington, said it was actively working with families to make sure their information was up to date, although it didn’t have current data on students who were not in compliance. Last year, 1,253 Lake Washington students claimed a personal exemption, according to Department of Health data.

Tacoma Public Schools said 368 students within the district are registered for a personal exemption and have not yet filed paperwork to show they are compliant.

The new law is rooted in the concept of “herd immunity,” which limits the spread of disease when a high percentage of people are vaccinated, indirectly protecting people that can’t be vaccinated for medical reasons. Supporters of the law hope that increasing the number of children who are vaccinated for measles, mumps, and rubella will help limit the spread of that disease.