More than 19,000 of New York City's first responders have yet to comply with Mayor Bill de Blasio's new COVID-19 vaccine mandate on the eve of its deadline.

As of Wednesday, 86% of the city's 378,000 municipal workers have received at least one dose, according to the mayor's office. That's only a 2 percentage point increase since the mayor first announced the mandate a week ago. The number includes education workers and public health care workers, who were already required to get the shot by early October.

The mayor has ordered all city workers to receive at least one dose by 5 p.m. Friday, eliminating the previous alternative of weekly testing. Beginning on Monday, city employees who have not received shots will be sent home without pay.

The relatively slow pace of vaccinations suggests a potential showdown between first responders — whose vaccination rates have been consistently among the lowest of city agencies — and the administration, one that could potentially affect response times to critical emergencies involving police, firefighters and public emergency medical service (EMS) workers, who are employed through the New York Fire Department (FDNY).

Only 68% of all FDNY employees have shown proof of at least one dose. Based on headcount estimates provided by the Independent Budget Office, that comes out to 5,700 unvaccinated employees. That includes EMS employees, who on their own have a 74% vaccination rate. Firefighters, meanwhile, are hovering at 64%, bringing down FDNY's overall average. On Wednesday, FDNY Commissioner Daniel Nigro said that as many as 20% of city fire companies could close on Monday due to staffing shortages.

The New York City Police Department employees are faring better, with 74% currently inoculated. That leaves an unvaccinated total of 13,563 civilian and uniformed officers. The NYPD is the biggest city agency after the education department.

The Department of Corrections has the lowest vaccination rate, that of 53%. But because of staffing shortages at the city's main detention facility on Rikers Island, correction officers were given until December 1st to get their shots.

"We can’t rule out the possibility of some operational disruption," said City Councilmember Mark Levine, who chairs the council's health committee. But he said that the sacrifice was worth it given the still severe status of the pandemic. The city is recording, on average, about 800 cases per day — but those numbers could rise again as New Yorkers spend more time indoors this autumn and winter.

"We’re not out of the danger zone yet," Levine said. "The downside of operation disruption is less worrisome than the prospect of tens of thousands of unvaccinated public workers as we head into the cold season."

Already, there have been reports of missed garbage pickups across the five boroughs, the result of sanitation workers not showing up to work in protest of the mandate. That department has only 67% of its workers immunized.

De Blasio has predicted that there would be a surge of vaccinations among city workers over the next several days, pointing to the pattern that occurred with the Department of Education and public hospital employees. In the 10 days prior to the vaccine deadline for school employees, city officials said 18,000 staff members got vaccinated.

Asked his morning briefing on Thursday if he was having any second thoughts about the mandate, the mayor resolutely responded no.

"We expected that a lot of the vaccinations would happen toward the end of the deadline. We also know a lot of people make the decision once they really realize that they're not going to get paid," de Blasio said. "That's just the human reality."

De Blasio said city agencies would address staffing shortages by adjusting shifts and assigning mandatory overtime. The city can also enlist the services of private ambulance companies. The mayor added that any city employee who was reassigned would have the necessary skills to do that job. "Any need will be filled with people who are properly trained and people who are ready to do the job," he said.

Several hundred municipal employees standing outside of Gracie Mansion in protest of the mayor's vaccine mandate.

But as the mayor spoke, several hundred firefighters staged a protest outside Gracie Mansion, warning of severe consequences to come.

“It takes 12 weeks of training to do our job, 16 weeks of training to do the job of a firefighter. It’s not like you can pull a guy in off the street," said Faye Smythe, president of the Uniformed Fire Alarm Dispatchers Benevolent Association. His union has 200 members, about a quarter of which he said had not been vaccinated.

He added that the mayor, who has about two months left to go in office, had thrown a "grenade" and was now "watching everybody scurrying like rats because of what he did."

In a television interview on WPIX11, Police Commissioner Dermot Shea said Thursday that as many as 800 NYPD staffers had been vaccinated in one day. Asked about potential shortages, he said, "It's very much a topic in the inner circle." He said he was preparing to assign double shifts as well as mandate overtime.

Attempts by unions to challenge the mandate in court have not been successful. On Wednesday, a state judge ruled against a request by the city's largest police union to issue an injunction against the mayor's mandate.

Gwynne Hogan contributed reporting.