The treatment could be lifesaving for many who cannot get protection from the vaccine, but confusion about the drug has made some doctors slow to prescribe it.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.Evusheld is a key piece of the Biden administration’s strategy to protect the more than seven million Americans with weakened immune systems.Ted S. Warren/Associated Press“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.Sasha Mallett was eager to find a dose of Evusheld, but many of the health workers she called had not even heard of it.Amanda Lucier for The New York TimesDr. 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 Coronavirus Pandemic: Key Things to KnowCard 1 of 3Mask guidance.