Sasha Mallett, Sue Taylor and Kimberly Cooley all have immune deficiencies that make them especially vulnerable to Covid-19, and all have tried to get the same thing: a new treatment that can prevent the disease in people who either cannot produce antibodies after receiving a coronavirus vaccine or cannot get vaccinated at all.

Ms. Cooley, a liver transplant recipient in Duck Hill, Miss., got the antibody drug, called Evusheld, from her transplant team at the University of Mississippi Medical Center with no trouble. But Ms. Taylor, of Cincinnati, was denied the treatment by two hospitals near her home. And Dr. Mallett, a physician in Portland, Ore., had to drive five hours to a hospital willing to give her a dose.

As much of the nation unmasks amid plummeting caseloads and fresh hope that the pandemic is fading, the Biden administration has insisted it will continue protecting the more than seven million Americans with weakened immune systems who remain vulnerable to Covid. Evusheld, which was developed by AstraZeneca with financial support from the federal government, is essential to its strategy.

But there is so much confusion about the drug among health care providers that roughly 80 percent of the available doses are sitting unused in warehouses and on pharmacy and hospital shelves — even as patients like Ms. Taylor, 67, and Dr. Mallett, 38, go to great lengths, often without success, to get them.

Because they have a weakened response to the coronavirus vaccine and may not be able to fight off Covid-19, many immunocompromised people have continued to isolate themselves at home and feel left behind as the country reopens. Evusheld, which is administered in two consecutive injections, appears to offer long-lasting protection — perhaps for half a year — giving it considerable appeal for this group.

For now, though, the drug is in short supply. Because it is authorized only for emergency use, it is being distributed by the federal government. The Biden administration has purchased 1.7 million doses — enough to fully treat 850,000 people — and had nearly 650,000 doses ready for distribution to the states as of this past week, according to a senior federal health official. But only about 370,000 doses have been ordered by the states, and fewer than a quarter of those have been used.

“There’s so many other people who are scrapping and driving for hours to get Evusheld,” Ms. Cooley, 40, said, “when in Mississippi it’s sitting on the shelves.”

Interviews with doctors, patients and government officials suggest the reasons the drug is going unused are varied. Some patients and doctors do not know Evusheld exists. Some do not know where to get it. Government guidelines on who should be prioritized for the drug are scant. In some hospitals and medical centers, supplies are being reserved for patients at the highest risk, such as recent transplant recipients and cancer patients, while doses in other areas of the country are being given out through a lottery or on a first-come, first-served basis.

Hesitance is also an issue. Some doctors and other providers do not know how to use Evusheld and are thus loath to prescribe it. The fact that it is an antibody treatment can be confusing, because most such treatments are used after someone gets Covid rather than for preventive care.

Adding to the confusion are revised Food and Drug Administration guidelines for Evusheld, released last month, that called for doubling the initial recommended dose after data showed the drug may be less effective against certain variants.

“It is overwhelming and it’s all new,” said Dr. Mitchell H. Grayson, chief of the allergy and immunology division at Nationwide Children’s Hospital in Columbus, Ohio. “Providers are definitely trying to keep up, it’s just — I don’t know how well everyone’s doing with that.”

Roughly 3 percent of Americans are characterized by health professionals as immunocompromised because they have a disease that weakens their body’s immune response or are receiving a treatment that does so. They include transplant recipients and people with conditions like cancer, lupus and rheumatoid arthritis.

Evusheld’s arrival in December immediately set off a scramble. In Facebook groups and online messages, patients and their loved ones began swapping information about how to get it. Government data sets about Evusheld’s availability were so complex and confusing that a software developer in the Seattle area, Rob Relyea, developed his own mapping tool that tracks how much of the drug is available and which providers have it.

“People should know where to go to get in line,” he said.

Mr. Relyea, 51, had a vested interest: His wife, Rebecca, is in remission from cancer. They tried 10 hospitals unsuccessfully but then got the drug through luck, as Ms. Relyea’s name was picked in a lottery for Evusheld at a hospital near their home in early February, he said.

But they have not heard anything yet about scheduling a second dose, which Ms. Relyea needs based on the new recommendations.

Dr. Mallett, in Oregon, was one of many who were desperate to get the drug. She has common variable immunodeficiency, a condition that keeps her immune system from making enough antibodies. Her son started attending kindergarten in person last fall, and when the Omicron variant surged, his teacher and classmates began testing positive for Covid.

To find Evusheld, Dr. Mallett scoured an online government database of shipments and spent weeks cold-calling hospitals, pharmacies and health organizations that received the drug.

When she finally found a hospital in La Grande, Ore., willing to give her a dose, she worked with her physician to enroll as a patient there. Then she dropped everything and drove to the hospital in the rain, received the shots and immediately turned back — an 11-hour trip in total.

Dr. Mallett is highly educated, medically savvy, wealthy and easily able to take time away from her job — privileges that helped her get a dose, but that many others do not have.

“I definitely have a lot of lingering ethical qualms about how I went about getting this medication,” she said. “Did I take advantage of our broken system?”

Many of the health workers Dr. Mallett called while she was trying to find a dose had not even heard of Evusheld — even if their workplaces had the drug in stock.

Some experts argue that Evusheld should go first to people who cannot get vaccinated because of severe allergies and to those who produce the fewest antibodies in response to coronavirus vaccines. But antibodies are only one component of the immune system, and the Centers for Disease Control and Prevention still recommends against using tests that determine antibody levels to assess someone’s immunity.

“The biggest problem is that there is absolutely no guidance or prioritization or any rollout in place at all, and it’s been a mess,” said Dr. Dorry Segev, a transplant surgeon at N.Y.U. Langone Health who has been studying coronavirus vaccines in transplant patients. “Without formal guidelines, you really can’t do anything.”

The Biden administration is trying to address the confusion. Top federal health officials have been working to raise awareness among state health officials, providers and patients. They convened a call this past week with advocates for the disabled to discuss the revised dosing guidance; they also urged patient groups to partner with the administration on outreach and education efforts.

“I feel really strongly that this medication has great potential to help the immune suppressed who don’t always respond to vaccinations,” said Dr. Rachel Levine, the assistant secretary for health in the Department of Health and Human Services, who spoke on the call. But Dr. Levine said she did not anticipate that the C.D.C.’s guidance on antibody tests would change.

Patients who cannot be vaccinated are obvious candidates for Evusheld. But among the vaccinated, Dr. Segev and other experts say, the calculations become far murkier — and can involve assessments of other underlying conditions or risk factors.

For patients who manage to get Evusheld, experts say it is still unclear exactly how much protection the medication offers. It is difficult to gauge the impact of the drug in protecting immunocompromised patients, because many recruited for studies were avoiding risky behaviors and it would have been unethical to ask them not to. Researchers may not know the exact effectiveness of the drug for many months.

Evusheld was found to offer protection comparable to vaccines in a clinical trial, but the number of participants who were immunocompromised was never disclosed. Adding to the uncertainty, AstraZeneca studied the drug before Omicron surfaced. Research over the past few months shows that Evusheld protects against the variant, but it is unclear to what degree.

The lack of solid information has frustrated Ms. Cooley, the liver transplant recipient in Mississippi. She is still taking the same precautions as she did before receiving Evusheld, such as getting groceries delivered, staying at home and seeing only a few trusted family members with masks on. That is because she cares for her elderly mother and has seen a number of other elderly people, including her grandmother, die from Covid-19 in her community, where many people have chosen not to get vaccinated.

Some who cannot find a dose of Evusheld have turned to online communities instead of health care organizations. They are seeking help from other immunocompromised people, such as Dr. Vivian G. Cheung, 54, a physician in Bethesda, Md., who has a genetic condition that affects her immune system.

Dr. Cheung got a dose in January after calling various medical institutions for two weeks, and she has been helping others navigate the process since then. She receives up to 10 requests for help every day, but she estimates that only a quarter of those who have reached out have succeeded in getting Evusheld.

Ms. Taylor, the woman in Cincinnati, has common variable immunodeficiency. But right now, one hospital near her is limiting its supply of Evusheld to its transplant patients, while another is not yet accepting patients from outside its system. She is unable to look elsewhere; she said she was uncomfortable driving long distances because of her underlying health conditions.

Ms. Taylor said that she did not want to take a dose away from someone who might need it more, but that she would feel less “panic-stricken” if she could get Evusheld. She might be able to start seeing her children indoors again and inch back to the life she had before Covid.

For now, she is in a holding pattern of isolating, masking and hoping a dose will become available soon.

Rebecca Robbins contributed reporting.