More and more students at Jewish schools in ultra-Orthodox Brooklyn went without the measles vaccine over the past five years, setting the stage for what’s become the largest measles outbreak in New York City in nearly 30 years.

The percentage of students who are unvaccinated for measles ticked steadily upwards from 2.1 percent in 2012 to 3.8 percent in 2017 among yeshivas in Borough Park and Williamsburg, an increase of about 1,177 children, according to a WNYC analysis of the most recently available state data.

While misinformation about the dangers of the vaccine has spread rapidly through social media, it doesn’t appear to have affected all communities at the same rate. In non-Jewish schools across the city, the rate of unvaccinated children have remained steady during that time: 2 percent of private school students, and 1 percent of public school students.

“[Parents] come up with their own ideas about when the children are healthy enough,” says Dr. Jay Begun, a pediatrician in the heart of South Williamsburg’s ultra-Orthodox community. “It used to be 10 years ago, most people would come in at two months and start giving the shots.”

Now, he says, parents often wait until the child is a bit older, and sometimes go without them altogether.

“A big argument that they use is, ‘Oh the baby’s immune system isn’t ready to receive the shots,’” he added.

(Clarisa Diaz, WNYC/Gothamist. Data analysis by Dave Sheingold.)

Earlier this month Gothamist/WNYC reported on a telephone hotline for Orthodox moms that was fanning fears of vaccines with misinformation. But experts on Judaism could not explain why the rate of unvaccinated children were higher in yeshivas than the general population. (Some other private schools also showed high rates of unvaccinated children, according to the state data, but they have far fewer students.)

“Deep down I think the mothers really believe [vaccination] is the right thing to do, but they’re nervous, they’re scared,” Begun said. “That has caused them just to delay it.”

Begun blames the debunked theory that vaccines cause autism and other misinformation about the dangers of vaccines, instead of religious reasons, for the hesitancy among his ultra-Orthodox patients.

“That has had an effect worldwide on immunization and we’re feeling the effects of that here in Williamsburg, Brooklyn,” he said.

Many religious and community leaders have publicly encouraged community members to get vaccinated and most interpretations of Jewish law also support it, according to Rabbi Mark Dratch, executive vice president of the Rabbinical Council of America.

“Because of the low-risk vaccines bring and the great benefits they bring to the individual and the community as a whole that there would be a Halachic Jewish...obligation to vaccinate,” he said. “The overwhelming majority of rabbinic authorities stand with the medical community.”

Rabbi David Niederman, head of the United Jewish Organizations of Williamsburg and North Brooklyn, an umbrella organization for the area’s Hasidic community, emphasizes that unvaccinated children are a small minority.

“The utilization rate of immunizations is extremely high in our schools,” Niederman said. “I saw the audits of the schools, very high—100 percent many of them.”

(Clarisa Diaz, WNYC/Gothamist. Data analysis by Dave Sheingold.)

Since the outbreak began in October, 7,000 more children were vaccinated in ultra-Orthodox areas than the year prior, according to the New York City Department of Health and Mental Hygiene, meaning the downward slump leading up to the measles outbreak will likely be reversed when the latest school year’s data becomes available.

But because an estimated 1,800 children still aren’t vaccinated, the department issued an order in December, prohibiting them from attending yeshivas in nine ZIP codes until the outbreak was declared over. (The ZIP codes roughly correspond to the neighborhoods of Borough Park and Williamsburg, Brooklyn.)

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About a fifth of the approximately 135 Jewish schools in those areas have 100 percent immunization rates for measles. Most schools fall somewhere between 90 percent and above. Another 15 schools have rates lower than 90 percent.

Vaccines are required by state law, unless parents provide proof of a religious or medical exemption. The city’s department of education evaluates the religious exemptions at public schools, while each individual private school decides whether or not to accept the child’s exemption, according to state Health Department spokeswoman Jill Montag.

An administrator at one private school said in an email she does not have control over her school’s immunization rate.

“When we accept a child into our school, we do not ask parents about immunization,” said Jennifer Chace, the director of the New Amsterdam School in the East Village, where just 60 percent of kids had measles vaccines in 2017. “As a part of the enrollment process, parents need to return the required medical forms and immunization records, and it is at that time that we would be made aware of a religious or medical exemption. Not before.”

Among the two dozen private schools with the lowest immunization rates, there are yeshivas, Islamic schools, Christian ones, and several non-religious ones, like Brooklyn Waldorf and the Rudolf Steiner Schools.

Still, the sheer size of the Jewish school system in Williamsburg and Borough Park, and the density of the ultra-Orthodox population in those areas make it a unique case. While the increase in the percentage of unvaccinated students was small, it is troublesome just the same, said Dr. Walter Orenstein, a professor and associate director of the Emory Vaccine Center in Atlanta.

“This is of concern,” he said. A cluster of unvaccinated children in one school, he added, allows the virus to maintain “an ongoing chain of transmission.”

“A decision that is made not only affects the individual child, but affects the community at large,” Orenstein said.

The results of clustering of unvaccinated children can be seen at Yeshiva Kehilath Yakov on Wilson Street in Williamsburg. Immunization rates had dipped there, from 97.1 percent in 2012 to 92.7 percent in 2017, leaving about 49 children unvaccinated at last count.

According to the city’s Health Department, one unvaccinated child who was infected with measles but wasn’t yet showing symptoms attended school despite the order. That child spread the virus to other unvaccinated kids who also shouldn’t have been there, resulting in an additional 42 measles cases, department officials said.

Two administrators at the Yeshiva Kehilath Yakov who declined to give their names told WNYC that a parent had sent an unvaccinated child on the school bus. The school tried to contact the parent to send the child home, the administrators said, but were unsuccessful. The school kept the child in a separate room and sent the child home on the school bus at the end of the day, they said.

“We fully comply with the Department of Health,” one of the administrators said. However, neither would answer follow up questions about the other unvaccinated children whom health department officials said were also in attendance.

Gwynne Hogan is an associate producer at WNYC. You can follow her on Twitter at @GwynneFitz.