New York City on Tuesday declared a health emergency following a measles outbreak in the Orthodox Jewish community in Brooklyn.

For months, New York City officials have been fighting a measles outbreak in ultra-Orthodox Jewish communities in Brooklyn, knowing that the solution — the measles vaccine — was not reaching its target audience.

They tried education and outreach, working with rabbis and distributing thousands of fliers to encourage parents to vaccinate their children. They also tried harsher measures, like a ban on unvaccinated students from going to school.

But with measles cases still on the rise and an anti-vaccination movement spreading, city health officials on Tuesday took a more drastic step to stem one of the largest measles outbreaks in decades.

Mayor Bill de Blasio declared a public health emergency that would require unvaccinated individuals living in Williamsburg, Brooklyn, to receive the measles vaccine. The mayor said the city would issue violations and possibly fines of $1,000 for those who did not comply.

“This is the epicenter of a measles outbreak that is very, very troubling and must be dealt with immediately,” Mr. de Blasio said at a news conference in Williamsburg, adding: “The measles vaccine works. It is safe, it is effective, it is time-tested.”

The measure follows a spike in measles infections in New York City, where there have been 285 confirmed cases since the outbreak began in the fall; 21 of those cases led to hospitalizations, including five admissions to the intensive care unit.

City officials conceded that the earlier order in December, which banned unvaccinated students from attending schools in certain sections of Brooklyn, was not effective. Mr. de Blasio said on Tuesday that the city would fine or even temporarily shut down yeshivas that did not abide by the measure.

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