When Jim Lewis was told earlier this month that his 90-year-old mother, who lives in a nursing home outside of Boise, Idaho, tested positive for Covid-19, he wondered if she had gotten the virus from an unvaccinated employee.

And he had reason to worry. A little more than half of the workers in the home, Creekside Transitional Care and Rehabilitation, were not vaccinated at the time, federal data show.

“It was obvious that the facility had staff members who were vaccine hesitant,” said Mr. Lewis, whose mother and immediate family are all immunized.

Idaho was hard hit by the Delta surge this summer and early fall, and nursing homes were not impervious to the highly contagious variant that swept through many states with lower vaccination rates. Ten states, including Florida, Michigan and Ohio, still report vaccination rates for nursing home staff under 60 percent.

Others, like New York and California, and some large nursing home chains have imposed their own mandates. But many nursing home administrators are waiting for the federal government to issue new rules that will govern a mandatory vaccination program for all their staff members that President Biden first announced two months ago. And some facilities and labor groups are still pushing for a testing option in lieu of a shot.

But months of delays and vaccine resistance have had wrenching consequences for families like Mr. Lewis’s, who once again are barred from visiting because of outbreaks. Creekside did not return repeated calls and emails seeking comment.

After steep declines earlier this year, Covid cases and deaths in nursing homes climbed in August and September, resulting in about 4,000 deaths — even though nearly 90 percent of the nation’s nursing home residents were fully vaccinated. Residents are particularly vulnerable to breakthrough infections because so many are older and suffer from serious medical conditions, like the multiple myeloma the former secretary of state Colin Powell was being treated for when he died from complications of Covid on Monday.

“It is medically wrong and borders on unethical to have unvaccinated nursing home staff caring for residents,” said Dr. Michael Wasserman, a geriatrician and former nursing home executive who has become a critic of the industry. “The vaccine works. It has made a profound difference.”

The Biden administration has said that nursing homes could face a loss of government funding — the industry heavily relies on Medicaid and Medicare funds — if they do not comply, but the regulations for enforcement of a mandate have yet to be disclosed.

Federal officials say they expect to issue the regulations sometime later this month. The rules were delayed from last month after the mandate was broadened to include all health care workers.

Officials from the Centers for Medicare and Medicaid Services, which will be issuing the rules, would not comment on the forthcoming regulations and did not make anyone available for an interview. It is unclear whether they are considering a testing option for nursing homes, according to people who have worked closely with the administration.

Residents and their families say they are frustrated with the monthslong delays in securing that extra layer of protection, given that so many of the unvaccinated are aides and nurses who provide the most direct, daily care.

Elizabeth, a nursing home resident in Minnesota, said she caught Covid earlier this year from an unvaccinated worker before she got her second dose of the Moderna vaccine. When she asked when the staff might be vaccinated under the president’s order, she was told the nursing home may focus on testing workers rather than requiring them to be vaccinated.

“Nothing has happened,” said the resident, who asked that only her middle name be used and that her nursing home not be identified in fear of retaliation, a concern shared by others interviewed for this article.

Her state’s Covid surge prompted the governor to call in the National Guard last week to help ease its severe shortage of health care workers.

“I just feel like a sitting duck,” she said. While she continues to wear a mask, some of the staff are no longer taking the same precautions to control the spread of the virus. “It just feels horribly unsafe,” she said.

Waning effectiveness of the vaccines that were first given to many nursing home residents has also raised concerns in the last few months.

“People are dying, residents are dying,” said Susan Reinhard, the director of the AARP Public Policy Institute, which has pushed for more transparency about vaccination rates in nursing homes. “They should be afraid.”

Lower vaccination rates translate into more infections, and mandates are a way to increase those rates, said Dr. Ezekiel J. Emanuel, a bioethicist at the University of Pennsylvania who advised Mr. Biden during his transition. “We should be clear that mandates have been working and have been working in every industry that has tried them,” he said.

Vaccination rates among nursing home staff increased to 69 percent by early October from 62 percent in early August, when Mr. Biden announced the mandate, but some facilities still report a staff rate of half or fewer, according to the latest federal data.

While some nursing homes have moved ahead with their own mandates, many are taking a wait-and-see approach, said Mark Neuberger, a lawyer with Foley & Lardner who advises health care organizations on employment issues.

“Will there be an alternative?” asked Zach Shamberg, the chief executive of the Pennsylvania Health Care Association, a state trade group representing nursing homes that is pushing to be able to test employees in lieu of the vaccine. “That is preferable to simply losing those workers, especially as providers are combating work force shortages.”

Many nursing homes “remain very concerned that we are not going to see vaccination acceptance rates increase,” he said.

“Our hope is that the small print allows us to do a test-out option,” said Mary Susan Tack-Yurek, the chief quality officer and a partner at Quality Life Services. The family-owned nursing home chain in western Pennsylvania reports that a little more than half of the staff is vaccinated. “Are we pleased with our staff vaccination rate? No, we are not pleased with it,” she said.

But if those workers left rather than be forced to get the shots, “we just couldn’t function,” Ms. Tack-Yurek said. “There aren’t enough resources in agency staffing, in the National Guard, to pull from other states, to make it up.”

The nursing home chain, which employs about 1,100 people, has been unable to persuade workers to get immunized, despite holding raffles offering rewards like a chance to go to Disneyworld or $5,000 in cash. “The response was minimal,” Ms. Tack-Yurek said.

Other nursing home officials dismiss the option of testing as an alternative. “We already have testing,” said Brendan Williams, the chief executive of the New Hampshire Health Care Association, whose members have been more successful in vaccinating their employees. “That is just preserving the status quo.”

Much depends on the communities where nursing homes draw their workers. If an area’s opposition to vaccination is strong, it becomes more difficult to sway recruits to get shots, and vice versa.

At Chaparral House in Berkeley, Calif., where vaccination rates are high, the vast majority of employees presented with paperwork to get immunized were willing, said the nursing home’s chief financial officer, Chuck Cole. “Most people didn’t read beyond the first paragraph,” he said, because they were already persuaded they should get the shot. “That was very important.”

By talking one-on-one with the small number of workers who were concerned about the vaccine, the nursing director and administrator were able to persuade the holdouts, he said. Only one employee of roughly 150 still refuses to get vaccinated.

Covid cases in the United States have dropped significantly in the last month, as more people are vaccinated and the Delta surge seems to be subsiding in most regions of the country.

The state mandates are helping to increase levels of protection for all age groups, and about 14 percent of the nation’s nursing home residents have already received a booster dose.

And some nursing homes that successfully imposed their own requirements are contributing to a higher success rate, said Brian McGarry, a health researcher at the University of Rochester who is studying levels in nursing homes. Genesis HealthCare, a large chain in Pennsylvania, said while there were some departures, all of its staff is now vaccinated.

“We are seeing the benefits of our policy in resident safety, as our Covid rates (and particularly the severity of any infections) have declined considerably since we instituted our policy, despite overall community infection rates remaining very high,” said Lori Mayer, a company spokeswoman, in a statement.

“The mandates are starting to help,” Dr. McGarry said. The actual enactment of a requirement by a state or facility “is a heavy lever and signal to this group that it’s not going to be optional any more.”