A measles outbreak in Los Angeles County, California, is centered on the Orthodox Jewish community, according to reports.

Twenty people have been infected so far, the Los Angeles Times reported on Saturday, at least 15 of them from the same social circle in Los Angeles county.

None of them could provide proof of vaccination, according to a spokesperson for the LA County Department of Public Health.

In late December Rabbi Hershy Z. Ten, president of Bikur Cholim in Los Angeles, was informed of the outbreak by the medical director for the Los Angeles Department of Public Health Immunization Program. He wrote an article in Jewish Home LA stressing the importance of vaccination.

“The disappointing fact that this latest outbreak is centered in the Jewish community is alarming, and speaks to the need for our leadership to take a proactive role in ensuring the health and safety of our children and families,” he wrote.

Six months ago, strict legislation came into effect in Californian lifting all previously approved religious exemptions from vaccinations. A child who has not been immunized is forbidden from attending kindergarten or school.

The county tracked down some 2,000 people who may have come into contact with the infected children and found that about 10 percent of them had also not been vaccinated.

Ten wrote that “in 2013 – 2014, there were as many as 21% of kindergarten students in some Jewish schools who were not immunized.”

Measles is caused by a virus and is highly contagious. There is no specific cure or treatment for measles and it causes among the most vaccine-preventable deaths of any disease, according to the World Health Organization. Most of those who die from the disease are under 5 years old.

However, immunization led to the disease being almost completely eradicated from North America.

Opposition to vaccinations among a segment of the US ultra-Orthodox Jewish community is led by a few rabbis, some of whose followers respect their views more than those of scientists and doctors.

Philadelphia-based Rabbi Shmuel Kaminetzky, one of the most influential ultra-Orthodox leaders in the US said in 2013 that “every individual retains the halachic right to choose whether to vaccinate or not vaccinate his children, in accordance with his concerns.”

Rabbi Shlomo Miller, the leading authority of Jewish law in Toronto ruled that “forcing someone to vaccinate his children against his will when the school is not compelled to do so by law, is against Daas Torah (the opinion of the rabbis).”

In 2014, Rabbi Moshe Tendler, one of the heads of New York’s Yeshiva University and an expert in medical ethics strongly condemned the view of these rabbis. “This is an area in which medicine has made such tremendous progress for the benefit of humanity,” Tendler told The Forward. “I believe that there may very well be rabbis who agree with Kaminetzky, but they are not speaking under their authority as rabbis, they are speaking simply as uninformed laymen.”