A federal appeals court delays New York City’s vaccine mandate for teachers.

Mayor Bill de Blasio's administration has been temporarily blocked from enforcing a vaccine mandate for nearly all adults in New York City public school buildings, after a federal appeals court granted a temporary injunction on Friday.

The mandate, which affects well over 150,000 people working in the nation's largest school system, was set to go into effect on Monday at midnight. Educators, parents and union officials have been bracing for the likelihood of staffing shortages and disruption in at least some schools where significant numbers of educators and staff members are not vaccinated.

A judge for the U.S. Court of Appeals for the Second Circuit granted the injunction on a temporary basis and referred the case to a panel of three judges for review. City officials said they expected the review and ruling to take place in the next few days, possibly even over the weekend, and anticipated that the mandate would be upheld. But it is not clear if the issue will be resolved before the Monday deadline.

Last week, a State Supreme Court judge ruled that the city could move forward with the mandate, after considering a separate but similar lawsuit filed by a coalition of unions that represents employees in public schools. The judge, Laurence Love, said state and federal courts have consistently upheld mandatory vaccination orders.

And on Thursday, a federal judge in Brooklyn, Brian M. Cogan, declined to grant the injunction sought by a group of teachers, calling the mandate "a rational policy decision surrounding how best to protect children during a global pandemic." The teachers then appealed, successfully, to the Court of Appeals.

At least 90 percent of teachers and 95 percent of principals are already vaccinated. The rate is lower — about 82 percent — among staff members in school buildings.

The leaders of the unions representing the city's teachers and principals have called on Mr. de Blasio to delay the implementation of the mandate, arguing that schools are not prepared to deal with staffing crunches.

The mandate, which was announced last month , requires all educators, along with staff like custodians, school lunch helpers and safety agents to receive at least one dose of a coronavirus vaccine by Monday night. It is the first vaccine mandate without a test-out option for any group of city workers.

"We're confident our vaccine mandate will continue to be upheld once all the facts have been presented, because that is the level of protection our students and staff deserve," a spokeswoman for the Department of Education, Danielle Filson, said in a statement.