Many parents and teachers are cheering New York City’s new vaccine mandate for public school staff, saying it makes them feel safer about classrooms reopening amid the rise of the highly contagious delta variant. But the mandate is also facing opposition. Some municipal unions are demanding officials bargain over the details such as penalties for non-compliance and are threatening legal action that could delay implementation.

Mayor Bill de Blasio announced Monday that all education department employees — including teachers, administrators, cafeteria workers and custodians — must show proof that they have received at least one dose of COVID-19 vaccines by September 27th, two weeks after classes resume. The mandate will apply to 148,000 education department workers. The announcement coincided with the U.S. Food and Drug Administration granting full approval to the Pfizer-BioNTech vaccine.

The new mandate goes further than the policy the mayor announced earlier this summer for all city workers, including teachers, that requires vaccination or weekly testing. It applies to the city’s traditional public schools and includes any charter schools that are co-located with them. The mandate does not apply to pre-k teachers at community-based organizations outside the city’s school buildings, bus drivers or private schools. The latter can set their own rules and are governed by the state.

“As a New York City public school parent, I am thrilled with the new vaccine mandate for all public school staff,” said Robin Morris, whose daughter is in middle school in Manhattan. “This mandate makes me feel safer for my kid and her school community.”

“Good news, finally,” elementary school teacher Liat Olenick tweeted. “As an immunocompromised transplant recipient teaching unvaccinated kids, this makes me feel safer and will make kids safer.”

But, with the new requirement hinting at stricter rules for the general workforce, the Municipal Labor Committee (MLC) voted Monday to take legal action and force the city to negotiate details of implementation, including penalties and medical exceptions. “Our members’ bargaining rights in this situation must be preserved,” Harry Nespoli, president of the MLC, which represents 350,000 city workers, said in a statement. The union is considering an injunction or lawsuit.

Crucial questions remain about what kind of consequences there will be for school staff members who refuse the shot in New York City. Under the vaccine-or-test mandate for the city’s municipal employees, workers could be suspended without pay for failing to comply. De Blasio said he would be “working through” the penalties with unions under the new policy for public schools.

“There will clearly be consequences if someone doesn't comply, unquestionably,” the mayor said Monday on MSNBC. “But our goal is to get people there. So, we're going to figure out the best way to convince people that this is the right time to do it.”

It is also unclear whether the vaccine mandate could add to staffing challenges at schools if a substantial number of employees opt to skip work rather than take the shots. De Blasio said he believes that people generally respond to the mandates, and he’s anticipating “overwhelming compliance.” Research shows vaccine mandates tend to be effective. But de Blasio added that the school system is ready to deploy substitutes as needed.

Other unions are voicing their concerns. The United Federation of Teachers released a statement Monday that appeared to accept the mandate, with exceptions.

“The city’s teachers have led the way on this issue, with the great majority already vaccinated,” said union president Michael Mulgrew. According to the city’s tally, at least 63 percent of school staff have received one or more doses of the vaccine, although that number only includes employees who were vaccinated within the five boroughs. Officials believe the total number of vaccinated staff is higher.

“The one thing I want to negotiate is individuals who have a legitimate reason from their doctors why they can’t be vaccinated,” Gregory Floyd, president of Teamsters Local 237 that represents about 5,000 school safety agents, school aides, and food service managers, told WNYC/Gothamist. “Everybody should be able to consult their doctor and be comfortable.”

De Blasio said on MSNBC the city would respond to employees’ medical concerns on a case-by-case basis. “If someone has a serious medical condition, that’s something we’ll work with doctors to address,” he said.

Clinical trials and real-world assessment of the vaccines so far have shown that the only medical condition that could prevent people from getting the shot is a severe allergy. This is the case for the Pfizer, Moderna, and Johnson & Johnson shots, and it has only occurred in 2 to 5 people per million who take the COVID-19 vaccines. Even in those cases, doctors can reverse the consequences with allergy meds.

But people cannot just seek this medical exemption simply because their bodies react to peanuts or shellfish. Not all severe allergies are the same, and in this case, exemptions will apply to people who react to specific ingredients in the vaccines.

Still, New York Teachers for Choice, a group that opposes the vaccine, has been mobilizing. “We simply want people to have [a] choice,” said Garrett Ramirez, a paraprofessional and member of the group. “We have civil liberties concerns,” he said, “and we do not have a great deal of faith in the CDC or the FDA.”

Some parents who have been lobbying for a remote option said the new requirement does not ease their fears about the city’s decision to bring students back into classrooms five days a week. “Vaccinations will not overnight erase fears that parents have,” said Farah Despeignes, president of the Bronx Parent Leaders Advocacy Group. “Vaccinated people can still contract the virus and [infect] others. Not all children and parents will be vaccinated.”

Breakthrough infections—cases where the virus bypasses vaccine-reinforced immunity—are sporadic, even with the delta variant. California, for example, releases weekly data on breakthrough cases. Its latest tranche still shows the vaccines are lowering the chances of infection by about 85%.

Yet studies suggest that only full vaccinations—two shots of Pfizer and Moderna vaccines or one shot of Johnson & Johnson—can protect against the coronavirus variants. New York City education staff who start the vaccine process with Pfizer and Moderna this week won’t be fully inoculated by the time school begins, given the time between doses and the two weeks it takes for immunity to kick in—but they would be covered in time with Johnson & Johnson's single-shot vaccine.

The mayor has said he does not anticipate requiring eligible students to be vaccinated at this point. Officials report 300,000 kids ages 12-17 in New York City—or 56 percent—have already been vaccinated with at least one dose, which is level with the national average for that age group. It is unclear how many of those students attend the city’s public schools.

With Monday’s announcement, the nation’s largest school system joined Los Angeles, Chicago, Washington state, and Oregon in requiring all school staff to be vaccinated. New Jersey Governor Phil Murphy also announced Monday that all public and private school employees must show proof of vaccination by October 18th or agree to regular testing.

“Now our parents will know, and all our employees will know, that every single employee in [schools] has been vaccinated,” de Blasio said on MSNBC Monday afternoon. “That's how we keep this school system safe, and we bring our kids back.”