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Through the fall, traveler after traveler arrived in the ultra-Orthodox Jewish communities of New York from areas of Israel and Europe where measles was spreading. They then spent time in homes, schools and shops in communities where too many people were unvaccinated.

Within months, New York State was facing its most severe outbreak of the disease in decades, with 182 cases confirmed by Thursday, almost exclusively among ultra-Orthodox Jews. Health officials in New Jersey have reported 33 measles cases, mostly in Ocean County, driven by similar conditions.

In 2018, New York and New Jersey accounted for more than half the measles cases in the country.

Alarmed, health officials began a systematic effort to bring up vaccination rates and halt the disease’s spread.

But while there has been progress, the outbreak is not yet over. Health officials said part of the problem has been resistance among some people in ultra-Orthodox neighborhoods to fully cooperate with health workers, get vaccinations and promptly report infections.