The New York City Department of Health has ordered mandatory measles vaccination within a section of the Borough of Brooklyn, where there have been more than 250 documented cases of the highly-contagious disease since September.
The New York Civil Liberties Union also questioned the city's move. This vaccine is safe.
The agency said it "continues to be seriously concerned about the accelerating numbers of measles cases being confirmed nationally".
The commissioner is empowered by law to issue such orders in cases when they might be necessary to protect against a serious public health threat.
Under the mandatory vaccinations, members of the City's Department of Health and Mental Hygiene will check the vaccination records of any individual who may have been in contact with infected patients.
The city's health commissioner, Dr Oxiris Barbot, said that the majority of religious leaders in Brooklyn's large Orthodox communities supported vaccination efforts, but that rates have remained low in some areas because of resistance from some groups that believed the inoculations were unsafe.
She also urged parents to avoid the practice of "measles parties", which she said has contributed to the outbreak.
As part of the declaration, every unvaccinated person living in the ZIP codes 11205, 11206, 11211 and 11249 - regardless of whether they have been exposed to the infection - are required to receive the measles-mumps-rubella vaccine to stymie the outbreak.
"When people choose not to get their children vaccinated, they are putting their children and others - such as pregnant women, people on chemotherapy, and the elderly - at risk of contracting measles", Herminia Palacio, NYC's deputy mayor for health and human services, said in the release.
Following that outbreak, the health commissioners office issued orders for all yeshivas in Williamsburg to exclude non-vaccinated children from attendance.
New York's mayor has declared a public health emergency following a measles outbreak.
But some experts also point to an "anti-vax" viewpoint that has gained some traction in recent years, helped by social media.
In Brooklyn and Rockland County, anti-vaccination advocates have been circulating a 40-page booklet around Orthodox Jewish enclaves.
While the MMR vaccine is the safest and most effective method of prevent measles, it is only 97 percent effective, so population-wide immunity is a key component to protecting our most at risk New Yorkers from measles.
'In general, the Orthodox community does immunize, ' said Dr. Joseph Kaplovitz, who works at the NYU Langone Stepping Stones Pediatrics clinic in Brooklyn's Borough Park. About a thousand developed encephalitis, which is swelling of the brain. Deaths are rare in the US, but measles killed 110,000 people globally in 2017. While it is possible to become infected with measles post-vaccination, the Mayo Clinic says it is unlikely, provided the immunizations are up-to-date.
The CDC recommends all children get two doses of the measles vaccine.
AP Medical Writer Mike Stobbe and Associated Press writers Robert Bumsted and Mary Esch in Albany, New York, contributed.