While the national vaccine mandate for health care workers remains mired in the federal courts, government officials say there is ample proof that requirements work — and do not cause a mass exodus of employees that some critics had feared.

Philadelphia, which issued a vaccine mandate in August, would seem to be a case in point. Virtually all hospital employees there are now fully vaccinated against Covid-19, according to a group of local hospitals that met last week with officials from the Centers for Medicare and Medicaid Services, which issued the national rule. About 95 percent of nursing home employees are also fully vaccinated, according to the city health department, compared with an average of 75 percent across the country.

With several states, including Pennsylvania, reporting sharp increases in Covid cases and hospitalizations and the arrival of the new Omicron variant, vaccine protection for health care workers is once again a critical tool.

Pennsylvania now has one of the country’s highest hospitalization rates. “Covid continues to rage on,” Dr. Jaewon Ryu, the chief executive of one of the state’s largest hospital groups, Geisinger, said at a news conference. Its intensive-care units are overwhelmed, he said.

In Philadelphia alone, 366 people were hospitalized in early December, compared with around 200 before Thanksgiving, according to city data.

The city’s hospitals and nursing homes were able to achieve high vaccination rates without significant staff departures. “At the end of the day, very few people quit,” Chiquita Brooks-LaSure, the agency’s administrator, said in an interview after the meeting.

Ms. Brooks-LaSure said it was important that hospitals and nursing homes educate their workers about the benefits of the vaccine and address safety concerns, although she added that local and workplace orders clearly influenced higher vaccination rates.

About 40 percent of the nation’s hospitals have vaccine requirements, and numerous states and cities imposed them for health care workers. In New York, which issued a mandate in late August, about 96 percent of hospital and nursing home workers are fully vaccinated, according to state data.

In California, which also required workers to get vaccinated, about 94 percent of nursing home employees have done so, and outbreaks have fallen, according to state data.

In states without mandates, like Oklahoma, about a third of nursing home workers are not fully vaccinated, and there have been thousands of recent cases among residents and staff.

At Crozer Health, a hospital group based in Springfield, Pa., the vaccination rate among workers climbed to 98 percent from 65 percent, largely because the area’s hospitals all had similar requirements and faced the looming federal mandate, Conlen Booth, Crozer’s senior director of emergency preparedness, said. “Staff understood there was no other alternative,” he said.

Crozer asked infectious disease specialists to directly address employees who were hesitant, including first responders like paramedics whose jobs did not typically put them in direct contact with someone who could answer their questions authoritatively. “They had never been spoken to by a physician,” Mr. Booth said.

Even nursing home executives acknowledged that the city’s requirement resulted in fewer departures than predicted. Philadelphia’s 47 nursing homes reported an average of about seven staff departures related to the vaccine requirement, according to the Pennsylvania Health Care Association, which represents the state’s nursing homes.

Zach Shamberg, the group’s chief executive, said Philadelphia remained “a cautionary tale” given that some employees did leave. Because a widespread staffing shortage is already hindering the ability of nursing homes to care for residents, “we cannot afford to lose one more worker,” he said.

There was some disagreement among hospital executives on the panel, which explored techniques employed to get all their workers vaccinated. One administrator warned that mandates could smack of “Big Brother,” while another defended the federal requirement as crucial leverage.

Officials from the Centers for Medicare and Medicaid Services also emphasized the unique role of individuals who care for the public, pointing to cases where patients were uncomfortable with nurses or aides who were not immunized.

“Health care workers have a special ethical and professional duty to protect the patients,” Dr. Lee Fleisher, the agency’s chief medical officer, said at the meeting with the hospitals. “There is no question that in any health care setting, the unvaccinated pose a direct and indirect threat to patient safety and, from our perspective, population health.”

The future of President Biden’s vaccine mandates for health care workers and private employees in other sectors is unclear. Federal judges have blocked the C.M.S. rule, which required health care providers to get their workers — an estimated 17 million — fully vaccinated by early January or risk losing federal funding.

The judge in the U.S. District Court for the Western District of Louisiana ruled that the agency had overstepped its authority by issuing an emergency regulation. He said the mandate “is something that should be done by Congress, not a government agency,” and added that “it is not clear that even an act of Congress mandating a vaccine would be constitutional.”

The administration has appealed, and the rule is on hold. “While C.M.S. remains confident in its authority to protect the health and safety of patients in facilities certified by the Medicare and Medicaid programs, it has suspended activities related to the implementation and enforcement of this rule pending future developments in the litigation,” the agency informed health care providers early this month.

The issue is likely to be decided by the Supreme Court, and the uncertainty has left hospitals and nursing homes “between a rock and hard place” in terms of whether to move forward with the requirement, said Katrina A. Pagonis, a lawyer specializing in regulatory issues at Hooper, Lundy & Bookman.

Both hospital and nursing home trade groups said they were encouraging vaccinations. “Providers should continue to prepare their policies and procedures and encourage unvaccinated staff to get their first shot,” Mark Parkinson, the chief executive of the American Health Care Association, which represents nursing homes, said in a statement.

Citing the widespread staffing shortages in his industry, Mr. Parkinson said his group would continue to pursue a testing alternative to prevent workers from leaving for other jobs.

In Philadelphia, hospitals are the largest employer, with some 57,000 workers, according to an estimate from Philadelphia Works, a nonprofit.

The city’s experience also underscored the work necessary to persuade all employees, particularly those of color, to get vaccinated, according to the hospital executives.

At the University of Pennsylvania Health System, the adoption of its mandate on Sept. 1 initially widened the gap between the vaccination rates for white and Black staff. In speaking with employees of color, doctors and administrators said they were explicit about the role race and racism played in decision-making and made a point to feature workers of color, including doctors, pharmacists and security guards, in their campaign. The system’s vaccination rate rose to nearly 100 percent from 70 percent.

While executives had been told that holding town halls was not effective, Dr. Florencia Greer Polite, an obstetrician, said she thought the issue was frequently the “all white” makeup of the panels.

“You have to have the right messenger,” agreed Michael Young, the chief executive of Temple University Health System, which worked with its union and community members. Main Line Health, another local hospital group, said it relied on “vaccine ambassadors” to talk to employees in departments like food service and housekeeping.

In Philadelphia, Ms. Brooks-LaSure said it was clear that hospital and nursing home executives had focused on addressing the concerns of the workers who were unsure about the safety of the vaccine. Women worried about the effect of the vaccine want to hear, “This is going to be OK for my baby, this is going to be OK for me,” she said.

She said these efforts were especially important in communities where there was strong distrust of the government because of past experiences. Latino residents, she said, may be wary of government programs because of the Trump administration’s attempt at a “public charge” policy that would deny immigrants legal status based on their use of government health programs.