Philadelphia will require all of its roughly 26,000 city employees to get a coronavirus vaccine by mid-January or risk losing their jobs, officials announced Friday.

With the new policy, Philadelphia joined a number of other cities with vaccine mandates and reversed a prior stance that unvaccinated workers simply had to wear two masks while at work.

“We bear a responsibility to mitigate the harm that would result from inadvertent transmission of COVID-19 to our colleagues and the public and to set an example for other organizations and companies,” Mayor Jim Kenney said in a statement. “We owe it to our city — and to ourselves — to do all we can to keep us all safe.”

Once the policy takes effect, Philadelphia will be among about a dozen U.S. cities with the strictest vaccine mandates in the country because they do not offer an option for weekly testing in lieu of vaccination. At least 12 other cities have mandates without a testing option, including New York, Chicago, Denver, Los Angeles, and Seattle, according to an analysis by Johns Hopkins University.

Philadelphia’s announcement came days after Health Commissioner Cheryl Bettigole expressed concern about a “sharp uptick” in new cases of the coronavirus and said that the increase, along with cold weather and holiday season, could mean another surge.

But the Kenney administration is already facing pushback from some employees. After the police union claimed the policy violated their contract, an arbitration panel on Friday evening blocked the mandate from taking effect for city police officers, pending further consideration.

While the city recently mandated that non-union employees, representing about 14% of the workforce, must be fully vaccinated by Dec. 1, the new policy will also apply to city contractors and workers who are represented by the city’s four labor unions.

Starting Jan. 14, the policy will allow for 15 days of unpaid leave for unvaccinated employees, and they will be terminated by the end of that leave if they have not gotten vaccinated. The city will offer religious and medical exemptions, but employees won’t be permitted to simply opt out. Employees who provide proof of vaccination by Dec. 24 can receive cash bonuses of up to $300.

The Fraternal Order of Police Lodge 5, which represents 6,000 police officers, has a clause in its contract approved in September stating that an arbitration panel will “resolve any disputes over the implementation” of a vaccine mandate. The FOP immediately sought arbitrators’ involvement Friday, and received a suspension of the mandate until the panel takes further action. (That suspension applies only to FOP members and not other city workers.) FOP President John McNesby also threatened further legal action.

“The FOP is conferring with counsel regarding potential litigation in local, state, or federal courts,” McNesby said.

City spokesperson Joy Huertas said the arbitration panel’s action was an expected step.

“We look forward to presenting our case to the arbitration panel and hope the FOP will join us in advocating for all their members to get vaccinated,” she said.

Local 22 of the International Association of Fire Fighters, the union that represents city firefighters and paramedics, issued a statement Friday assuring members they would not be disciplined for resisting the mandate.

“We will use every resource” to advocate for members, union leadership wrote in a statement.

Like the FOP, the union said it has carved out some contractual protections for unvaccinated members. The union reached a tentative three-year contract agreement last month that stipulates a neutral arbitrator will hear disputes over vaccine mandates, according to the statement.

The union further assured members they wouldn’t be disciplined or placed on unpaid leave for resisting the mandate “without input from an arbitration panel.” Nonetheless, the union urged all members to submit their vaccine status by the end of November.

The city’s Police and Fire Departments both have vaccination rates for employees between 51% and 61%, according to city data. Only the Streets Department has a lower rate, between 41% and 51%.

City spokesperson Kevin Lessard said the administration is having ongoing discussions with municipal unions about the new policy.

“We will continue to work closely with city departments and our municipal unions to monitor employee compliance with the mandate and ensure no impact to core city services,” Lessard said. “We know, from the experience of other municipalities and organizations, that mandates work and the vast majority of impacted employees comply with such mandates.”

Mandates have been instituted in other cities over threats and protests from labor unions. In New York, about 9,000 of the city’s 370,000 municipal employees were put on unpaid leave when the policy took effect earlier this month and about 12,000 workers requested exemptions that the city needed to review, according to the New York Times. And while hundreds of firefighters called in sick to protest, officials reported no disruption to city services.

Not all unions in Philadelphia have been resistant. The American Federation of State, County and Municipal Employees District Council 33, the largest union for city workers that represents blue-collar workers including sanitation workers and crossing guards, signaled support for a vaccine mandate, with some conditions, over the summer.

AFSCME District Council 47, which represents the city’s white-collar workers, could not be reached for comment Friday.

The Philadelphia Streets Department has the lowest vaccination rate, between 41% and 50%, according to city data. The Fire Department has a vaccination rate of between 51% and 60% as of this week, and all other departments have at least 60% of workers vaccinated.

» READ MORE: Only a third of Philly city employees and half of Pa. state health workers have reported being vaccinated

To apply for religious or medical exemptions, workers must submit a request to the Employee Relations Unit in the city’s Department of Labor by Dec. 20 with documentation from a health-care provider or certification explaining why religious beliefs prevent vaccination.

“Philosophical or moral exemptions are not permitted,” the policy states.

Philadelphia already has a vaccine mandate in place for health-care and higher-education employees. But Bettigole had previously stood by the city’s decision not to require vaccination for all city employees. Instead, workers were required to wear two masks at work if they had not voluntarily reported their vaccination status to the city.