NEW YORK — Even among New York’s Hasidic Jews, members of the ultra-Orthodox Satmar sect are known for their strict religious and cultural traditions. They speak mainly Yiddish. They shun the secular world. They are skeptical, if not suspicious, of anyone from outside their insular community in Brooklyn’s Williamsburg neighborhood

Now the refusal of some parents to vaccinate their children – a decision not based on any religious proscription – and the resulting measles outbreak have brought public health authorities to their doorsteps in a collision of cultures could turn messy.

Most of the measles cases in the Williamsburg neighborhood of Brooklyn, N.Y., have involved children. Sharon Pulwer for The Washington Post

On Wednesday, the city sent 15 to 20 “disease detectives” into the community, some with Yiddish translators, a day after Mayor Bill De Blasio’s vow 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 virus and checked the immunization records of all those they may have come in 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 students with religious exemptions in the four ZIP codes targeted by the city, according to spokeswoman Marcy Miranda.

Anyone deemed at risk and unable to produce proof of immunization by the end of Wednesday faced a fine, she said, adding that no one would be immunized involuntarily.

“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.

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

“We’re talking about a very small minority who aren’t vaccinated,” agreed Israel Friedman, a property manager who said his many siblings and two of his three children have been immunized. His infant child 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, so-called “herd immunity” protects all but an unlucky few.

“It certainly can’t be 98 or 99 percent,” said John Marshall, chairman of emergency medicine at Maimonides Medical Center, the major 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, said Rabia Agha, director of pediatric disease. Still, the girl’s parents refused to have her vaccinated after she recovered, despite the entreaties of the medical staff.

On another occasion, Marshall said, he had to threaten to call police on a couple who were refusing to send a feverish child to the hospital in an ambulance out of 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’ infectious disease division.

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

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

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

Measles, considered eradicated from the United States in 2000, is not just a fever and a rash. It can cause pneumonia as well as encephalitis, an inflammation of the brain that can have long-term consequences. Before the widespread use of vaccines beginning 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 is 95-97 percent effective, said Jeffrey Avner, chairman of the Maimonides department of pediatrics.

Measles is contagious from four days before the appearance of the telltale rash and until four days after, so exposures often occur 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 Officer Joseph Deutsch acknowledged that any call from an agency like 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.”

At least some of the misinformation in the ultra-Orthodox Jewish community comes from a 40-page pamphlet produced by an 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, raises questions about the morality of vaccines.

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

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

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 other illnesses.

Citing the loud rabbinical support for vaccinations, Niderman said he would be surprised if the city had to fine anyone in his community for flouting the city’s 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 by 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.”