A COVID-19 vaccine requirement for teachers and other staff members took effect Monday in New York City’s million-plus-student public school system, in a key test of the employee vaccination mandates being rolled out across the country.

The city had planned to bring in substitutes where needed, with Mayor Bill de Blasio warning that unvaccinated school employees would be placed on unpaid leave. But 95% of the city’s roughly 148,000 public school staffers had received at least one vaccine dose as of Monday morning, including 96% of teachers and 99% of principals, de Blasio said.

“Our parents need to know their kids will be safe,” the mayor said. “They entrust us with their children. That’s what this mandate is all about. Every adult in our schools is now vaccinated, and that’s going to be the rule going forward.”

Schools Chancellor Meisha Ross Porter said she did not know exactly how many employees had declined the shots and been put on leave.

Implementing the mandate smoothly could be a test for de Blasio, a Democrat who boasted of the city’s record of keeping school buildings open during most of the last school year when other districts went to all-remote instruction. New York City is not offering a remote option this year.

The nation’s largest school system is one of the first in the country to require inoculations for all its staffers. The system’s mandate does not include a test-out option, but does allow for medical and religious exemptions. A similar mandate is set to go into effect in Los Angeles on Oct. 15.

Some 43,000 doses have been administered since the mandate was announced Aug. 23, de Blasio said.

U.S. Secretary of Education Miguel Cardona hailed the vaccine mandate, saying New York City is “doing it right.”

“Students need to be in the classroom. They need to be safe and we need to make sure we’re doing everything possible to let our staff get vaccinated and make sure that our schools are as safe as possible,” Cardona said.

Vaccination rates rose in every school job category after the mandate was announced. District Council 37 of the American Federation of State, County and Municipal Employees, which represents some 20,000 city Department of Education employees, said 93% of those workers had provided proof of at least one COVID-19 vaccine dose as of Sunday, up from 68% at the beginning of September.

Under a deal worked out with the city, unvaccinated District Council 37 members who want to apply for a medical or religious exemption have until 5 p.m. Tuesday to do so, union officials said in a news release.

The mandate was supposed to go into effect last week, but was delayed when a federal appeals court granted a temporary injunction sought by a group of teachers. An appeals panel reversed that decision three days later. A request to the Supreme Court for an injunction was also denied.

Roxanne Rizzi, who teaches technology at an elementary school in Queens, waited until Friday to get her first coronavirus vaccine shot.

“I had to do it for the finances of my family,” she said.

Rizzi, 55, had resisted the vaccine because she contracted COVID-19 in November and believed natural immunity would protect her. She said she would continue to protest the mandate.

According to the U.S. Centers for Disease Control and Prevention, people should get vaccinated even if they have already been infected by the virus. The agency says COVID-19 vaccines offer better protection than natural immunity and help prevent getting infected again.

Some people vocally opposed to vaccine mandates did protest Monday. Video posted to social media showed demonstrators marching in locations around the city, including outside a Department of Education building in Brooklyn, carrying signs saying things such as: “Resist Medical Tyranny!”

Another video on social media showed people overturning a COVID-19 testing facility’s tent and table as the protesters marched passed it.

Many students and parents support the vaccine mandate as the best way to keep schools open during the pandemic.

“It’s safer for our kids,” said Joyce Ramirez, 28, who was picking her three children up from a Bronx elementary school last week.

Cody Miller, a 15-year-old sophomore at a high school in Manhattan, said teachers should all be vaccinated. “I think they should,” said the teen, who got vaccinated himself as soon as the Pfizer shot was approved for people 12 and up. “It’s so many kids, it’s a big environment, you know?”

But Mally Diroche, another Bronx parent, had mixed feelings. “I kind of feel like that’s a decision they should be able to make on their own,” said the mom of three boys between 3 and 12. Diroche, 29, said she feels that masks and other precautions can check the spread of the virus within schools.

The COVID-19 vaccines authorized for use in the U.S. were tested in tens of thousands of people and proven to be both safe and effective at dramatically reducing the risk of serious disease and death. The vaccines now have been given to more than 200 million Americans and that real-world use plus extra government safety tracking have made clear that serious side effects are extremely rare — and that any risk is far lower than the risks posed by COVID-19.

Correction: This story has been corrected to show that 43,000 is the estimated number of vaccine doses administered since Aug. 23, not the number of employees who have been vaccinated.