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NEW YORK, NEW YORK - JULY 22: Melissa Moy, a teacher at Yung Wing School P.S. 124, goes over a lesson on a monitor with in-person Summer program students on July 22, 2021 in New York City. Positive COVID-19 cases in some New York City public schools have resulted in classroom quarantines. (Photo by Michael Loccisano/Getty Images)

(CNN) -- New York City Mayor Bill de Blasio announced Monday that all Department of Education staff for public schools across the city must be vaccinated against Covid-19, without a testing alternative.

The landmark step, which came less than an hour after the US Food and Drug Administration granted full approval to the Pfizer/BioNTech Covid-19 vaccine, makes New York City the largest public school system in the US to mandate vaccines.

The mayor had previously announced that the city's entire municipal workforce, including teachers, must get vaccinated or go through weekly testing by September 13. With this new mandate, though, all school staff are required to provide proof of vaccination of at least one dose of the Covid-19 vaccine by September 27, the mayor said.

The NYC Department of Education employs 143,000 people and serves 1.1 million students in over 1,800 schools within the education system. Citywide, about 76% of adult residents have had at least one dose of a Covid-19 vaccine, according to city data.

The head of the United Federation of Teachers, the labor union that represents most NYC public school teachers, said in a statement the details of the mandate must still be negotiated.

"Our first priority is keeping our kids safe and the schools open. The city's teachers have led the way on this issue, with the great majority already vaccinated," UFT President Michael Mulgrew said.

"While the city is asserting its legal authority to establish this mandate, there are many implementation details, including provisions for medical exceptions, that by law must be negotiated with the UFT and other unions, and if necessary, resolved by arbitration."

The teacher vaccine mandate is particularly important because children under age 12 are not yet eligible to be vaccinated. Vaccinating all teachers would help create an immune shield around these unvaccinated students and help ensure schools can remain open and safe this coming school year.

The idea of a vaccine mandate is to provide a metaphorical stick that officials hope will push teachers to get vaccinated or face economic consequences. Still, the mandate will be difficult to implement and could lead to worker shortages if enough educators refuse the shot.

Under de Blasio, New York City has been at the forefront of requiring the Covid-19 vaccine. Earlier this month, NYC began to require proof of vaccination to enter all restaurants, fitness centers and indoor entertainment venues, though enforcement won't begin until next month.

"If you're unvaccinated, unfortunately, you will not be able to participate in many things," de Blasio said then. "If you want to participate in our society fully, you've got to get vaccinated."

The vaccine mandate is also part of de Blasio's vaccine-focused strategy at this phase of the pandemic. New York City has recommended mask wearing but stopped short of issuing an indoor mask mandate, a contrast from other major metropolitan areas like Washington DC and Los Angeles County.

"The overwhelming strategic thrust is vaccination, so we thought the right mix was to heavily focus on vaccination, continue to climb the ladder with more and more vaccine requirements in place, and to give a very clear message to all New Yorkers strongly recommending mask usage regardless of vaccination status," de Blasio said in early August.