This summer, amid the worst measles outbreak in decades, New York passed a law that says children who are not vaccinated can’t attend classes. It went into effect this week, reports BuzzFeed.

The mandate, enacted in June, “prohibits a school from permitting any child to be admitted to such school, or to attend such school, in excess of 14 days without sufficient evidence that the child has received all age appropriate required vaccinations,” according an official publication from the New York Department of Health, Office of Children and Family Services, and the State Education Department.

State health officials told BuzzFeed News that 26,217 unvaccinated New York children in public schools, private schools, parochial schools, daycare centers, and prekindergarten programs claimed religious exemptions during the 2017–18 school year. New York ended religious exemptions for vaccines in June, following the worst measles outbreaks the state had seen in decades.