Ten states sue the U.S. over the vaccine mandate for health care workers.

After a steady decline in infections earlier this year among nursing home staffs and residents, spikes in cases this past summer alarmed officials. Many nursing homes had large numbers of workers who remained unvaccinated even after Mr. Biden announced the plan to mandate immunizations.

Many medical societies came out in favor of strict mandates for health care workers, arguing these employees have a special obligation to keep their patients and colleagues safe. And many large, multistate hospital systems and large nursing home companies began requiring staff vaccination, although others lobbied against blanket requirements.

The administration said about 40 percent of all hospitals already require vaccinations. About 73 percent of nursing home workers are now vaccinated, according to federal data.

“It’s critical to us to make sure we’re ensuring the safety of residents living in nursing homes and other individuals in health care settings,” said Chiquita Brooks-LaSure, the administrator for Medicare, in an interview after the agency issued the new rule. Vaccinated staff members are less likely to get sick and spread Covid, she said.

Ms. Brooks-LaSure acknowledged providers’ concerns over losing workers who refuse to be vaccinated, but she said mandates often ease shortages because employees don’t become infected. “What we’re seeing on the ground is that they are not going to work because they are sick,” she said.

She also cited the experience in states that have issued requirements as evidence that vaccination rates will rise as a result of the government’s decision. Many large hospital systems and even city agencies that mandated the vaccine reported that only a small minority of employees were unwilling to be vaccinated.

But the 10 states — Alaska, Arkansas, Iowa, Kansas, Missouri, Nebraska, New Hampshire, North Dakota, South Dakota and Wyoming — claim the federal government has overreached its authority to dictate what happens in their states. “This case illustrates why the police power over compulsory vaccination has always been the province of — and still properly belongs to — the states,” they argue in the lawsuit.