The city took the unusual step amid a surge of 285 measles cases in the city since September, most in one densely packed neighborhood where people now have to get vaccines or risk a $1,000 fine.
In the U.S., the Centers for Disease Control and Prevention reported there have been 465 cases so far this year, two-thirds of them in New York state. That compares to 372 cases in the U.S. for all of last year. Besides New York, there have been outbreaks this year in Washington state, California. Michigan and New Jersey.
The disease was declared eliminated in the U.S. in 2000, which means it was not being spread domestically.
But cases have been rising in recent years, in part the result of misinformation that makes some parents balk at a crucial vaccine.
Most of the reported illnesses are in children. The CDC says roughly 80% of the U.S. cases are age 19 or younger.
Here are some questions and answers about measles:
Q: How dangerous is measles?
A: Measles typically begins with a high fever, and several days later a characteristic rash appears on the face and then spreads over the body. Among serious complications, 1 in 20 patients get pneumonia, and 1 in 1,000 get brain swelling that can lead to seizures, deafness or intellectual disability.
While it's rare in the U.S., about 1 in every 1,000 children who get measles dies, according to the CDC.
Q: How does it spread?
A: By coughing or sneezing, and someone can spread the virus for four days before the telltale rash appears.
The virus can live for up to two hours in the air or on nearby surfaces. Nine of 10 unvaccinated people who come into contact with someone with measles will catch it. Dr. Anthony Fauci, infectious disease chief at the National Institutes of Health, recently called it "one of the most contagious viruses known to man."
Q: Is a problem outside of the U.S.?
A: Measles is far more common around the world — the World Health Organization said it claimed 110,000 lives in 2017. The WHO says there's been a 30 % increase in measles cases in recent years. Unvaccinated Americans traveling abroad, or foreign visitors here, can easily bring in the virus.
For example, a huge outbreak in Madagascar has caused more than 115,000 illnesses and more than 1,200 deaths since September. But you don't need to go as far as Madagascar — common tourist destinations like England, France, Italy and Greece had measles outbreaks last year. Nearly 83,000 people contracted measles in Europe in 2018, the highest number in a decade.
Q: How many U.S. children are vulnerable?
A: Overall about 92% of U.S. children have gotten the combination vaccine that protects against measles, mumps and rubella, known as the MMR vaccine. Two shots are required, one around the first birthday and a second between age 4 and 6. Full vaccination is 97% effective at preventing measles.
But the CDC says 1 in 12 children do not receive the first dose on time, and in some places vaccination rates are far lower than the national average. For example, an outbreak in Washington state is linked to a community where only about 80% of children were properly vaccinated.
Q: Is the vaccine safe?
A: Yes. In the late 1990s, one study linked MMR vaccine to autism but that study was found to be a fraud. Later research found no risk of autism from the vaccine.
Q: Why isn't everyone vaccinated?
A: Some people can't be immunized for medical reasons — including infants and people with weak immune systems — and most states allow religious exemptions. But while vaccination against a list of contagious diseases is required to attend school, 17 states allow some type of non-medical exemption for "personal, moral or other beliefs," according to the National Conference of State Legislatures.
In Washington state, lawmakers are debating ending that personal or philosophical exemption, as are several other states. California ended a similar exemption in 2015 after a measles outbreak at Disneyland sickened 147 people and spread across the U.S. and into Canada.
Q: Why so many cases in New York's Orthodox Jewish communities?
A: Most families in Brooklyn's Orthodox enclaves do have their children vaccinated, and most rabbis say there is no religious reason not to get them. But anti-vaccine propaganda has found an audience among a larger than usual percentage of parents in a community used to cultural clashes with city officials. It is also a community whose members travel frequently to other countries where measles is more prevalent.
AP Medical Writers Lauran Neergaard in Washington and Mike Stobbe in New York contributed to this story.
The Associated Press Health & Science Department receives support from the Howard Hughes Medical Institute's Department of Science Education. The AP is solely responsible for all content.