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.