An unvaccinated student was permitted to attend a New York City religious school last fall.
Gizmodo reports that student has been linked to 21 other cases of the virus, in the worst outbreak in decades.
The infected student was not yet exhibiting symptoms, and attended class at the Yeshiva Kehilath Yakov school in Brooklyn.
This, despite health advisories that individuals who had not received the MMR vaccine should not be at school.
The school is situated in Williamsburg, where 87 confirmed cases have been reported since October.