How many health care workers are vaccinated? It’s anyone’s guess.

U.S. officials still don’t know exactly how many hospital workers remain unvaccinated, a blind spot that makes it difficult for public health officials to predict and assess vulnerabilities at facilities already facing staffing crises.

The lack of reliable immunization data, more than a year after vaccines were first made available to health care workers, could most immediately complicate Biden administration efforts to get ahead of a surge, or assess how many federal personnel might be needed in a region and prop up overwhelmed hospital systems.

Without more precise data, “you don’t know what’s happening and you don’t have the ability to say how at risk is the health care workforce,” said Celine Gounder, an infectious disease expert at New York’s Bellevue Hospital who advised President Biden's Covid-19 transition team. “It makes it harder to plan.”

The Supreme Court last week ruled that the administration’s vaccine mandate for almost all health care employees may take effect, but state and federal officials can’t say exactly how many workers the rule will cover nationwide, or how many of them would face penalties for not complying.

The data is more sparse when it comes to boosters, which the Biden administration recommends but isn’t mandating.

“This is just one of many data system failures we’ve seen,” said Larry Levitt, executive vice president for health policy at the Kaiser Family Foundation. “If you had to pick a place in a society where you want to know how many people are vaccinated, it’s hospitals ... These are critical essential workers caring for people who, by definition, are sick and more vulnerable.”

As of the end of December, about three in four hospital workers — 77.6 percent — were fully vaccinated, the Centers for Disease Control and Prevention said, but those figures, which include non-medical staff such as custodial and cafeteria workers, come from only about 40 percent of the nation’s hospitals. About four out of five nursing home staffers are fully vaccinated, according to the CDC.

Nursing homes have been required to submit weekly data since last May and roughly 90 percent do so, according to an agency spokesperson.

Hospitals, however, won’t have to submit data until May 15 under a federal rule released by CMS in August.

That kind of insight could be useful now as health care facilities around the country are swamped with the latest Covid surge and are increasingly relying on the National Guard and federal personnel to alleviate overwhelmed staff, many of whom are out sick with Covid. Unvaccinated staff are likelier to be out sick for longer, with more severe disease. In recent weeks, some hospital executives have said they have no choice but to bring infected and symptomatic staff back to work.

For now, the CDC has received figures only from hospitals that volunteer the information, according to the agency spokesperson. The CDC doesn’t have the authority to require hospitals to do so; that authority lies with CMS, the spokesperson added. A spokesperson for the White House’s Covid-19 response declined to comment.

Amesh Adalja, senior scholar at the Johns Hopkins Institute for Health Security, said he found it problematic that the majority of the country’s hospitals haven’t shared their vaccination figures.

“I suspect that many hospitals do not want to report their worker vaccination rates because they are very suboptimal and it is embarrassing,” he said in a Twitter message to POLITICO. “Perhaps they don’t want their peers, competitors, and patients to know that they employ health care workers who exercise poor clinical judgment.”

Many hospitals haven’t submitted immunization figures partly because they are overwhelmed with other data requests from federal and state governments, according to Janis Orlowski, chief health care officer of trade group Association of American Medical Colleges.

However, she added, vaccination rates “are now becoming an important number and probably the attention should turn to that.”

Health care facilities, including hospitals, nursing homes, hospices and other providers that accept payments from CMS, must comply with the vaccine mandate or risk losing access to Medicare and Medicaid funding.

Under the CMS requirement, all staff at health care facilities, such as hospitals, nursing homes and hospices, must be fully vaccinated in the next two months or risk losing federal funding from Medicare and Medicaid — big payers for the industry.

CMS can take action in the coming weeks, though the administration isn’t likely to cut a facility’s funding if it’s “working in good faith” to comply, said Sean Marotta, a lawyer advising the American Hospital Association.

Orlowski said the CDC’s data, while incomplete, is likely representative of the entire country. The AAMC conducted a survey of 125 academic hospitals several months ago and found results which roughly line up with the CDC’s, she said.

More than 99 percent of doctors and close to 90 percent of nurses were vaccinated, she said. The vaccination rate dropped off substantially — in the 30 to 40 percent range — for those in more operational roles, such as transportation and food service workers, she said.

Roughly half of the states put their own mandates in place requiring all health care workers to be vaccinated prior to the Supreme Court decision. In the other states, which are mostly controlled by Republicans, vaccination rates among health care employees tend to mirror the local population, according to health care analysts. Some Republican governors have either prohibited vaccine requirements or made them impractical to enforce.

Florida’s health agency has said in recent days that it won’t enforce the federal law — and that health care facilities must provide several exemptions to anyone wishing to evade the shot. A CMS spokesperson didn’t respond when asked how the agency planned to move forward in the state. Texas is the only state where the mandate doesn’t currently apply, CMS said, due to a separate court injunction.

A CMS spokesperson said the agency “is moving full speed ahead” on enforcing its vaccine requirement and that the agency will “have a sense of the overall vaccination rate” after its surveyors determine hospitals’ compliance. CMS didn’t say when it expected that to be.

The CDC has struggled to gather accurate and timely data throughout the pandemic, particularly when it comes to tracking how rapidly the virus is spreading, drawing concerns from scientists and some lawmakers.

Earlier this month, during a panel with CDC director Rochelle Walensky, Sen. Mitt Romney (R-Utah) expressed dismay about a several-week lag in Covid-19 figures.

“Do we need to invest in either developing a new subagency or a task force to get ... immediate data, daily data so we know what’s going on?” he said. “That’s not just for the public; that’s for those of you making decisions.”