In the face of the state’s worst outbreak of measles — a potentially deadly infectious disease — in decades, New York City has declared a public health emergency.

On Tuesday, Mayor Bill de Blasio announced at a news conference that unvaccinated individuals in certain Williamsburg zip codes would be required to receive the measles vaccine. The disease arrived in the area after an unvaccinated local child acquired it during a trip to Israel. Since then there have been 228 confirmed measles cases in the majority-Orthodox Jewish community, in what NBC reports is one of the largest outbreaks in decades.

“This is the epicenter of a measles outbreak that is very, very troubling and must be dealt with immediately,” the mayor said, claiming that the city would issue violations and potentially fines for those who didn’t abide by requirements. While he admitted that it was “unusual” for the city to go to such lengths, he said it’s “a reflection about the seriousness of this disease.”

This declaration comes just one day after the city’s health department barred unvaccinated children from going to class in local yeshivas, and two weeks after health officials in Rockland County, which lies approximately 40 miles north of New York City, declared a state of emergency, with 170 confirmed cases of measles in the area. On March 26, the county banned unvaccinated children from everywhere from shopping malls to restaurants to schools for 30 days — a controversial order that was lifted by a judge this past Friday when he ruled in favor of numerous parents who took issue with the ban.

Like Williamsburg, Rockland County has a large community of Orthodox Jews, many of whom are unvaccinated. (Children in tight-knit communities like this are especially susceptible to outbreaks.) While such communities are not necessarily tight-knit due to religious reasons, that happens to be the case in both Williamsburg and Rockland County, which has raised the question of how such policies as have been enforced in the city and upstate align with New York’s religious exemptions, which families can claim against vaccination requirements.

“It violates several Constitutional provisions,” Patricia Finn, a lawyer representing a Rockland family that claimed such an exemption, told CBS New York of the ban. “This is executive order in invalidating the religious exception is having a huge financial impact on families that now have to homeschool their kids [sic].”

However, in response to this specific issue, some Hasidic rabbis have been personally asking parents to vaccinate their children against measles.

“Everyone is obligated to do whatever it is in our ability to stop the spread of this disease,” read one statement from leading Rockland rabbis.