On Wednesday, the city sent 15 to 20 “disease detectives” into the community, some with Yiddish interpreters, a day after Mayor Bill de Blasio (D) vowed to quash the outbreak with $1,000 fines and misdemeanor charges for anyone in certain areas who refuses to be immunized.

The workers, wearing blue Health Department jackets, conducted interviews in the homes of people who may have been exposed to the dangerous, highly contagious measles virus and checked the immunization records of all those they may have had contact with. Others pored over records for the same information at a federally funded health clinic in the heart of the community. There are 1,800 unvaccinated yeshiva, or Orthodox Jewish, students with religious exemptions in the four Zip codes targeted by the city, spokeswoman Marcy Miranda said.

“It is highly unusual for the city to deploy health workers in this manner,” Miranda said. The last time health-care workers undertook the time-consuming work of contact tracing was during the Zika outbreak in 2016.

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New York City has had 285 cases, virtually all of them in Brooklyn, since the outbreak began in October. Of those, 229 were reported this year, accounting for nearly half of the 465 cases that have been reported nationwide in 2019. Now, measles has been found in more than a third of U.S. states — up and down both coasts, and across the plains, the Midwest and the South — with most of the illnesses in children.

In Williamsburg, the attention is becoming a sore spot for some in a community that would rather be left alone.

David Oberlander, principal of a yeshiva where measles was found earlier in the outbreak, criticized De Blasio’s office and what he called “very, very inaccurate stories” about his community. “Three percent are anti-vaccination,” he said. “Ninety-seven percent of our students and family are vaccinated.”

Israel Friedman, a property manager who said his many siblings and two of his three children have been immunized, agreed, saying: “We’re talking about a very small minority who aren’t vaccinated.” His infant is still too young to be vaccinated, he said.

Health authorities say they are seeing a very different reality. When 90 percent of a population is vaccinated against a particular threat, “herd immunity” protects all but a few.

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“It certainly can’t be 98 or 99 percent,” said John Marshall, chairman of emergency medicine at Maimonides Medical Center, the main hospital in the area. “If there were 90 percent of people immunized in the community, it wouldn’t be spreading.”

In fact, Marshall and other medical officials called the 285 known cases a severe undercount of the real measles toll in Williamsburg and its vicinity. Seven children and two adults have been hospitalized at Maimonides since the outbreak began, including one adult and one child in intensive care.

The child, a 13-month old, had breathing difficulties as a result of the measles, said Rabia Agha, director of pediatric disease. Still, the girl’s parents refused to have her vaccinated against other diseases after she recovered, despite the entreaties of the medical staff.

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Another time, Marshall said he threatened to call police on parents who were refusing to send a feverish child to a hospital in an ambulance for fear the authorities would learn all their children were unvaccinated.

“The ones who are so vehemently anti-vaccination, I don’t know how to convince them,” said Edward Chapnick, director of Maimonides’s infectious-disease division.

Public health experts warn that the city’s use of emergency power, while reasonable, could further alienate the ultra-Orthodox who already isolate themselves from greater society.

“When an outbreak is concentrated in a specific group, there is a risk of outsiders stigmatizing that group,” said Saad Omer, an infectious-disease expert at Emory University who researches public health and immunization. “This risk is exacerbated when a public health emergency is declared.”

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But with only a week until the Jewish Passover holiday, when families gather in large groups and travel to see relatives and friends, city officials thought they had to move quickly to try to stop the spread of the disease.

Measles, considered eliminated from the United States in 2000, leads not only to a fever and a rash, it also can cause pneumonia as well as encephalitis, an inflammation of the brain that can have long-term consequences. Before the widespread use of vaccines began in 1963, it infected millions every year in the United States, killing several hundred.

One dose of the vaccine is considered more than 90 percent effective at protecting against the virus; the recommended two doses are 95 to 97 percent effective, said Jeffrey R. Avner, chairman of the Maimonides Department of Pediatrics.

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Measles is contagious from four days before the appearance of the telltale rash and until four days after, so exposure often occurs without people realizing, especially during flu season, when many children show similar symptoms.

At the ODA Primary Health Care Network clinic on Heyward Street, chief executive Joseph Deutsch acknowledged that any call from an agency such as the city Health Department makes patients nervous. But the clinic understands that the only way to end the outbreak is to trace and test everyone who may have been exposed.

“We call those people and tell them they have to come in,” he said. “Does everyone come in? No. But most people are coming in.”

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At least some of the misinformation in the community comes from a 40-page pamphlet produced by the anti-vaccination group Parents Educating and Advocating for Children’s Health (PEACH), whose members are largely anonymous. Although rabbinical leaders in the community have urged vaccinations as consistent with Jewish law, one article in the pamphlet, published under a pseudonym, challenges the morality of vaccines.

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Another questions how much money doctors make by administering what the author describes as unnecessary care.

PEACH also runs a hotline, with archived conference calls, and 140 lectures by speakers it describes as doctors who do not believe in vaccinations.

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Rabbi David Niderman, executive director and president of the United Jewish Organizations of Williamsburg, expressed anger about the pamphlets spreading fear by putting out false information linking immunizations to autism and illnesses.

Citing the strong rabbinical support for vaccinations, Niderman said he would be surprised if the city had to fine anyone in his community for flouting the order.

“I don’t think they’ll ever get to that,” he said, “and we believe that those few who have not complied until now will fall in place.”

He said he has found a foolproof way to handle periodic calls from people upset about what they’ve read in the pamphlets, who ask him to reconsider his pro-vaccination message.

“I listen politely,” he said. “And then at the end . . . I say, ‘Mrs. So and So, I want to ask you one question: If your child needs emergency surgery, do you go to the person who tells you things anonymously? Or do you go to your family physician to get a referral to a good surgeon?’ In every case, the conversation stops there.”