MONSEY, N.Y. - In a suburban shopping center an hour north of New York City, hundreds of mostly ultra-Orthodox Jews gathered in a sex-partitioned ballroom to hear leaders of the national anti-vaccine movement.

Sustained applause greeted Del Bigtree, a former television producer-turned-activist who often wears a yellow star of David, similar to those required of Jews in Nazi Germany, to show solidarity with parents ordered to keep unvaccinated children at home.

Bigtree described the purported dangers of childhood vaccines in phrases that also conjured the Nazis.

"They have turned our children into the largest human experiment in history - all of history," he said.

The turnout last week in this suburb hard hit by measles helps explain why New York has become Ground Zero in one of this country's largest and longest-lasting measles outbreak in nearly 30 years. Even in a religious community grappling with more than 700 cases in Rockland County and New York City since last fall - among them, children on oxygen in intensive care units - anxious and confused parents said they came because they are afraid of vaccines and seeking guidance about what to do.

Ethan, a 36-year-old father of six from Queens who declined to give his last name, said he attended the event out of "a genuine concern" for his family, driven by his wife's research into vaccines. She had read "a lot of literature" and watched Bigtree's film, which accuses the government of covering up a purported link between the measles vaccine and autism - a tie repeatedly disproved by studies around the world involving hundreds of thousands of children.

As a result, Ethan said, measles frightened him far less than what Bigtree and others described as the toxic substances in vaccines.

"I love doctors," Ethan said, but they have "blind obedience" to the vaccine schedule. "God gave us a wonderful, beautiful body that heals itself."

State and national health officials say groups such as Bigtree's are directly responsible for the measles outbreaks that struck Orthodox communities here and in New York City this year. Through an aggressive social media campaign, pamphleting and traveling road shows that pop up in receptive and often insular communities, officials say, the anti-vaccine movement has produced pockets of unvaccinated children where the highly contagious and sometimes deadly disease can catch fire.

The groups' claims are flatly contradicted by science, but their rhetoric has sent vaccination rates plummeting across the country, including among Eastern European immigrants outside Portland, Oregon, the Somali community in Minnesota and ultra-Orthodox Jews in Brooklyn, and here in Rockland County - all groups that have seen recent outbreaks.

"This is a national movement of people who are nothing but charlatans, conspiracy theorists and people . . . spreading misinformation," said Rockland County Executive Ed Day. "The type of propaganda they spread is a danger to the health and safety of children within our community and around the world."

In many ways, vaccines are a victim of their own success. Years ago, people were intimately familiar with the suffering caused by diseases such as polio, whooping cough and measles. Today, they've been virtually eliminated - along with the memory of their terrible effects.

As a result, generations of parents have grown up "more likely to be scared of the vaccine than the disease," said Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia. "It's very easy to appeal to those fears."

The modern anti-vaccine movement began about 40 years ago in response to legitimate concerns about the side effects of a pertussis vaccine. But it has metastasized into something far darker in the echo chamber of Facebook chat rooms, WhatsApp and YouTube - especially against a backdrop of rising suspicion of elites, including drugmakers, doctors and public health officials.

Anti-vaccine activists have a rhetorical advantage: They speak with absolute certainty about frightening cases of so-called vaccine injuries based on changes parents say they observe in their children after getting shots.

Scientists and researchers, by contrast, rarely speak in absolutes. They say vaccines save countless lives, but like all medicines, have side effects - albeit rare ones. And they sometimes challenge what parents think they have seen with their own eyes by explaining that health problems such as autism often become apparent around the same time children are receiving multiple shots - even though there is no causal connection.

* * *

The modern anti-vaccine movement began with the heartbreak of a mother in Virginia in 1980 that would propel her into activism.

Several hours after her eldest son - then 2 1/2 - got his fourth shot to prevent diphtheria, pertussis (whooping cough) and tetanus, Barbara Loe Fisher said she found him staring straight ahead as if he couldn't see her. "When I called out his name, his eyelids fluttered, his eyes rolled back in his head, and his head fell to his shoulder," she recalled.

Fisher has written on her website that her son suffered a convulsion, collapse and brain inflammation and grew up with multiple learning disabilities. After seeing a television special on possible dangers of the DPT vaccine, Fisher suspected a link to the vaccine.

At the time, the pertussis component of the vaccine was made with many more proteins than other childhood vaccines, and had a significant risk of side effects, including fever and in some cases, seizures. (The problem was corrected in newer versions of the vaccine.)

Fisher became a national advocate, warning parents about possible risks and working with Congress to craft legislation creating a vaccine compensation program and an improved vaccine monitoring system - one of several safety systems still in use.

Today, the National Vaccine Information Center in Sterling, Virginia, which she founded, is considered one of the most effective lobbyists for parental choice, combating efforts in New York and other states to make it harder for parents to opt out of vaccinating their children.

"We don't tell people what to do," Fisher said in a recent interview. "We support informed voluntary medical decisions that people make. We do not tell people to vaccinate or not to vaccinate."

* * *

In 1998, the Lancet, a respected British medical journal, published a paper that would cause the anti-vaccine movement to explode. The paper, by gastroenterologist Andrew Wakefield and other authors, claimed to have found a link between the measles, mumps and rubella vaccine (MMR) and autism in eight children.

The paper was later found to be fraudulent. Wakefield's fellow authors issued a retraction. So did the Lancet after an investigation by British medical authorities. Wakefield was stripped of his medical license after a panel concluded that he had financial and ethical conflicts of interest and had acted "dishonestly and irresponsibly."

But the damage was done. MMR vaccination rates plunged in Britain, Ireland, the United States and other countries at a time of rising concern about autism diagnoses.

To many parents, Wakefield's thesis seemed believable because symptoms of autism first appear when children are about 12 months old - the same age they receive their first MMR vaccine, said Alison Singer, a New York City mother with a severely autistic daughter.

"Up until Wakefield, no one had really put the two together," said Singer, who now heads the Autism Science Foundation, which supports research into the condition's causes.

Twenty-one studies since that Lancet study have found no relationship between the MMR vaccine and autism. The latest and largest, from Denmark, involved 657,461 Danish children born between 1999 and 2010.

In the intervening years, researchers have implicated genetic and environmental factors in autism, such as older fathers and infections during pregnancy. Scientists now believe that more than 100 genes affect an individual's risk for autism, said Josh Gordon, director of the National Institute of Mental Health.

"About 15 to 20 percent of people with diagnoses of autism can now be told by their doctors why they have it," Gordon said.

Despite all the subsequent research, Wakefield's discredited ideas have become firmly entrenched in anti-vaccine mythology.

"Once you put a scary thought in someone's head, it's very hard to get it out," Singer said.

The notion that vaccines are implicated in autism and a host of other medical conditions is now championed by an increasingly organized anti-vaccine movement that includes at least a dozen national organizations and hundreds of Facebook groups, many of them private. Many cast themselves as promoting individual and parental rights and fighting government overreach - a cause that resonates with individuals across the political spectrum.

High-profile leaders such as Bigtree, founder of Informed Consent Action Network in Austin, and environmental attorney Robert F. Kennedy Jr., the nephew of president John F. Kennedy, crisscross the country railing against vaccine dangers and advocating for parental choice.

Bigtree, the son of a minister, is a charismatic speaker who draws large crowds at wellness conferences and state legislative hearings. A former daytime television producer for "The Doctors," he said he became a vaccine safety advocate after hearing from so many aggrieved parents after partnering with Wakefield on a movie about Wakefield's theories.

"The moment we began expanding the vaccination program, our health has been declining in our children," Bigtree said recently on a weekly live show he distributes on Facebook and YouTube. He says he has more than 140,000 Facebook followers and more than 50 million video views.

Kennedy's interest in vaccines grew out of his advocacy work on environmental pollutants.He accuses drugmakers of colluding with the health establishment to cover up vaccines's alleged role in rising rates of a gamut of chronic diseases and even teen suicide - claims rejected by the American Academy of Pediatrics and virtually every leading health and science organization in the world.

In January 2017, Kennedy stunned the medical establishment by announcing that then-President-elect Trump had asked him to lead a commission looking at vaccines and autism - a subject Trump had mentioned repeatedly on the campaign trail.

But the White House never went forward with the commission. And last month, when U.S. measles cases reached a record high, Trump urged parents to "get the shots" for their children.

In an extraordinary public rebuke this month, three members of Kennedy's family - including his brother, a former congressman, and his sister, the former lieutenant governor of Maryland - accused Kennedy of being "part of a misinformation campaign that's having heartbreaking - and deadly - consequences."

"We love Bobby . . . We stand behind him in his ongoing fight to protect our environment," they wrote. "However, on vaccines he is wrong."

* * *

Most Americans continue to support immunizations, as evidenced by high national vaccination rates, but there are worrisome trends: The percentage of children younger than 2 who haven't received any vaccinations, for instance, has quadrupled since 2001, according to the CDC.

And in more than a dozen hot spots across the country, including the Seattle and Portland areas, which have had measles outbreaks, immunization rates have plunged asan increasing number of parents receive nonmedical exemptions to avoid having to give their children the shots, according to a study last year.

The link between places with low vaccination rates and measles outbreaks is clear: In some of Williamsburg's yeshivas, for instance, up to 22 percent of children did not receive the MMR vaccine for religious reasons during the 2017-2018 school year, according to New York state data.

Anti-vaccine activists encourage that trend by arguing that the vaccine is potentially more dangerous than the disease.

At the Monsey forum, Rabbi Hillel Handler called resistance to measles shots a brave act. The virus, which once killed several hundred Americans every year, "is not a serious disease," he said, adding that those who battle it in childhood grow up stronger.

Public health officials are trying to fight back. They note that children who've recovered from measles are more susceptible to infections and are at risk for serious complications.

In the Brooklyn neighborhoods at the heart of the New York City outbreak, nurse practitioner Blima Marcus holds regular meetingswith small groups of ultra-Orthodox women in their homes, spending hours answering their questions. As a member of the same Orthodox community, Marcus says it is easier for her to gain their trust.

Often, she said, the women are surprised by the scientific studies she brings that disprove links between the measles vaccine and autism. Afterward, some tell her they feel they've been "really misled."

"These are insular women who are trying to do the best for their children," Marcus said. "At the end of the day, I feel the majority of people who don't vaccinate are the victims of a one-way propaganda machine."

The Washington Post's Lenny Bernstein contributed to this report.

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