Large numbers of public employees across the country are off the job because of surging coronavirus cases. Cars lined up for drive-through virus tests on Wednesday near the Getty Center in Los Angeles.Credit...Sarah Reingewirtz/The Orange County Register, via Associated Press

In Dallas, 204 of the roughly 2,100 total employees of the city’s fire and rescue department were in quarantine on Thursday because of positive coronavirus tests — the most since the beginning of the pandemic, according to Jason Evans, a department spokesman. He said that approximately one-quarter of the department’s total positive tests since March 2020 had taken place in the last two weeks.

In New York City, on any given day this week, 21 percent of subway operators and conductors — about 1,300 people out of a work force of 6,300 — have been absent from work, according to the Metropolitan Transportation Authority, which attributes the jump in absenteeism to the virus. Service has been suspended on three of the system’s 22 subway lines and schedules reduced on many others.

The mayor of Cincinnati recently declared a 60-day emergency to address what he described as a “public danger” posed by depleted public-safety staffing.

Los Angeles city officials said at a news conference on Thursday that almost 300 firefighters were off duty because of the virus, the most the department had seen at any one time. As recently as mid-December, the figure was 24. The Los Angeles Police Department said it had 505 officers out as a result of the virus.

“The Omicron variant has taken off like wildfire,” Mayor Eric Garcetti of Los Angeles said at a news conference on Thursday.

Officials said that the city had authorized extra overtime and had canceled all previously approved leave for Fire Department employees to cover the absences. The mayor said the police and fire departments “have maintained staffing needed to keep Angelenos safe.”

The rapid surge of coronavirus cases across the country, propelled by the highly transmissible Omicron variant, has snarled school reopenings and forced restaurants and other businesses to close for lack of workers. And the recent strain on police, fire and other public agencies has intensified disputes over vaccination mandates.

In Los Angeles, officials said thousands of police and fire employees had requested medical or religious exemptions to the city’s vaccine mandate. Some have since complied with the rules and gotten shots, but others are still in the midst of lengthy reviews. Some workers who did not get vaccinated by the deadline have been sent home without pay while they go through disciplinary processes.

Mr. Garcetti said that about 83 percent of the city’s police officers and firefighters were in compliance with the vaccine mandate. He urged employees, and all city residents, to get booster shots as well.

In San Francisco, where rates of new coronavirus case reports are at their highest since the start of the pandemic, public health officials have urged residents to “layer their defenses” by getting booster shots and wearing masks, in part to help ensure that 911 calls are answered quickly and buses and trains run on time.

“San Francisco is in a relatively good position, compared to other municipalities,” the city’s director of health, Dr. Grant Colfax, said in a statement. “But the Omicron variant is challenging us, even more than Delta, to manage this disease while keeping our economy, schools and other essential services open.”

Jeff Cretan, spokesman for Mayor London Breed, said on Thursday that 140 Fire Department employees and 188 Police Department employees had tested positive or were out because of quarantine protocols; so were 110 workers at the city’s transit agency.

Mr. Cretan said that having all of the city’s frontline workers be fully vaccinated had staved off worse trouble. “People aren’t getting sick and ending up in the hospital and dying from our work force,” he said.