NEW YORK (NYTIMES) - Through the fall, traveller after traveller 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 177 cases confirmed by Tuesday (Jan 15), 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 neighbourhoods to fully cooperate with health workers, get vaccinations and promptly report infections.

"Sometimes they hang up and they don't want to open the door," said Dr Patricia Schnabel Ruppert, health commissioner of Rockland County, north-west of New York City, where the worst of the outbreak has been, with 114 confirmed cases.

"It's hard to break an outbreak if you are not getting cooperation."

Dr Ruppert said that health officials discovered that some religious schools, or yeshivas, in ultra-Orthodox communities in Rockland County had vaccination rates as low as 60 per cent, far below the state average of 92.5 per cent. Audits found that some schools were overreporting vaccination rates, she added.

Delayed vaccination also helped fuel the outbreak in the Orthodox communities of Williamsburg and Borough Park in Brooklyn, which had reported 55 cases as of last week, said Dr Jane R. Zucker, head of the city health department's Bureau of Immunisation.

There have been no deaths in the outbreak, but there have been a few serious cases in young children that required hospitalisation.

Measles is one of the most contagious infections and can live for up to two hours in the airspace where an infected person breathed, coughed or sneezed. It usually affects children, and symptoms include high fever and a rash of red spots all over the body, as well as a cough and runny nose. Some 90 per cent of unvaccinated people exposed in proximity to an infected person will get it.

But the vaccine, when given in two doses - typically around age 1 and age 5 - is about 97 per cent effective.

Health officials and sociologists say the reasons for low vaccination rates among the ultra-Orthodox are complex.

In part they are tied to the wider anti-vaccination movement globally, including concerns that the measles vaccine, which also protects against mumps and rubella, causes autism or other diseases. The idea has been widely debunked but persists in some circles.

Rabbi Yakov Horowitz, founder of Darchei Noam yeshiva in Monsey in Rockland County, said that some parents considering admission to his school agonised over giving their children vaccines because they had heard they were dangerous. His yeshiva insisted on them, he said, though he knew of others that did not.

"Good people, great parents were terrified," he said. "They felt that I was asking to give their children something that would harm them."

Ms Alexandra Khorover, general counsel for Refuah Health Centre, one of the largest health providers in the Rockland community of Spring Valley, said her health workers had encountered "a small pocket of people who are anti-vaccine who have been peddling this information, fostering confusion and fear".

Part of the reluctance to vaccinate or allow a government health worker to enter the home, though, is cultural.

Dr Samuel Heilman, a Queens College sociology professor who studies the ultra-Orthodox, said that there is a "fear of interference from the outside" rooted in the community's origins in pre-World War II Europe. More recently, the ultra-Orthodox have fought back against other health department efforts, such as New York City's efforts to limit a controversial circumcision practice, metzitzah b'peh, because of warnings from health officials that it causes herpes in infants.

"They have accepted the idea that they live by different rules than others in the outside community," Dr Heilman said.

While this insularity allowed the measles to spread, it has also had a protective effect on wider public health, at least so far. In part because ultra-Orthodox Jews tend to attend their own religious schools and patronise their own shops and restaurants, the disease has remained in Orthodox circles, save for several infections among non-Jewish workers linked to their communities, health officials said.

The outbreak in New York and New Jersey can be traced to the rise of measles in Israel, where some 2,700 cases and two deaths were reported in 2018, centred in Jerusalem.

In Europe, which was the source of at least some of the Brooklyn infections, some 65,000 cases were reported in the year ending October 2018, with high concentrations in Balkan countries and Ukraine.

As measles spread in New York, public health officials swung into action. Some 40,000 flyers were printed in English, Yiddish, Spanish and other languages warning of the Israeli outbreak and calling for people to be vaccinated. Health officials met with rabbis and pediatricians, who sounded the alarm to their congregations and patients.

"We are telling people the health department is looking out for your health," said Rabbi David Niederman, a community leader and executive director of the United Jewish Organisations of Williamsburg. "They are the experts and you should take the vaccinations."

In Rockland County, which includes the large ultra-Orthodox community of New Square, authorities put 59 schools under "exclusion orders", forbidding unvaccinated children to attend even if they had a valid religious or medical exemption to the vaccine. The orders are lifted when a school's vaccination rate reaches 95 per cent, which state authorities consider protective of public health. Eighteen schools have had the orders lifted, officials said.

In Brooklyn, some children have been out of school for months because of similar exclusion orders by health officials, said Rabbi David Zwiebel, executive director of Agudath Israel, an ultra-Orthodox umbrella organisation. Tensions are high, with some parents still refusing to vaccinate because of health fears, and others relenting.

"There has been some harsh language exchanged on both sides," Mr Zwiebel said.