This public health emergency declaration comes after the NYC Health Department issued Commissioner's Orders last week to all yeshivas and daycare programs serving the Orthodox Jewish community in Williamsburg, doubling down on their order to exclude unvaccinated students or face violations and possible closure, first announced in December.
De Blasio, a Democrat, said officials were confident the order would withstand legal scrutiny.
The city's largest outbreak since 1991 of the once virtually eradicated disease has mainly been confined to the Orthodox Jewish community in the borough's Williamsburg neighborhood, with 285 cases confirmed since October, Mayor Bill de Blasio said at a news conference. None proved fatal, but 21 patients required hospitalization and five were admitted to intensive care. Those who are uninsured will pay what they can afford, de Blasio said, and those who cannot afford the vaccination will receive it for free.
The health order applies to anyone living, working or going to school in four zip-codes in the neighborhood and requires all unvaccinated people at risk of exposure to the virus to get the jab, including children over six months old.
The Brooklyn outbreak is part of a broader resurgence in the USA, with 465 cases reported in 19 states so far this year, according to the Centers for Disease Control and Prevention (CDC). They blamed the spike partly on anti-vaccine campaigns spreading misinformation that immunizations are unsafe.
Officials also noted that Passover is approaching, meaning increased travel among people who could carry measles to or from NY.
Michigan's measles outbreak dates back to March 13, when the health department reported that the first case came from someone "visiting from Israel following a stay in NY".
More than 280 people in Brooklyn and Queens have come down with measles since September.
On Friday, a NY state judge lifted the state of emergency imposed by Rockland County that would have barred minors not vaccinated against the measles from public places.
That message was also echoed by Dr Herminia Palacio, deputy mayor of health and human services, who said that "this vaccine is safe".
'We will make sure that everybody who is allowed will be vaccinated, ' Rabbi David Niederman, the president of the United Jewish Organizations of Williamsburg, said in a phone interview.
Population-wide immunity is important to stop measles because the vaccine isn't 100 percent effective. Once people have become infected by measles, their bodies build up their immune systems to prevent new inflections. It lays out numerous anti-vaccine arguments that have been debunked by scientific studies, such as a claim that measles, mumps and rubella vaccinations increase the risk of autism.
The Centers for Disease Control and Prevention recommends that all children get two doses of measles vaccine.
The National Institutes of Health says reports of serious reactions to vaccines are rare: about one in every 100,000 vaccinations.