The Food and Drug Administration has greenlit a new medicine to protect some of the people most at risk from Covid.

The agency granted emergency use authorization for Pemgarda, a monoclonal antibody infusion, in immunocompromised people ages 12 and older. The drug is intended to protect against Covid for people who are not likely to mount an adequate immune response after vaccination. This includes those who have received stem cell or organ transplants and cancer patients taking medications that suppress the immune system.

“It’s going to be for a very small section of Americans,” said Dr. Michael Mina, a former Harvard epidemiologist who is now the chief science officer for eMed, a telehealth company. But, he said, it’s a vital group to protect: the people who most feel left behind at this stage in the pandemic.

“For people who are immunocompromised and literally defenseless because their immune system doesn’t really work, this is really an important development,” said Dr. Ziyad Al-Aly, the chief of research and development at the Veterans Affairs St. Louis Healthcare System.

Pemgarda will become available within the next week or two, said David Hering, the chief executive of Invivyd, the company that makes it. The company is still evaluating what the drug will cost, Mr. Hering said; he expects that Medicare and private insurance plans will cover it.

The drug is given as an infusion in doctors’ offices and other health care settings; it takes about an hour to complete the infusion. The most common side effects in a clinical trial included reactions at the infusion site, cold and flulike illness, fatigue, headaches and nausea. Four out of 623 participants in the trial experienced anaphylaxis, a severe allergic reaction.

The goal is to give people the drug before they encounter the virus, to reduce the risk of severe outcomes from Covid and lower the chance of contracting the virus at all. “You think about it like vaccines,” Dr. Al-Aly explained. The drug is not a treatment for those who have Covid: You cannot take Pemgarda if you currently have the virus or were recently exposed.

People may opt to get a dose of the drug as often as every three months, Mr. Hering said. It’s not clear whether some people will have to get additional doses of the drug indefinitely, said Dr. Joseph Bailey, a pulmonologist with the Northwestern Medicine Comprehensive COVID-19 Center.

Pemgarda is not yet fully approved. “They haven’t taken it all the way across the finish line with their study yet,” Dr. Bailey said. The agency authorized the drug based on data that gauged people’s titers, a measure of antibodies, and so it’s too soon to assess exactly how effective the medication will be at preventing infections in the real world.

Previous monoclonal antibody treatments, like Evusheld, were removed from the market as they became less effective against new Covid variants. The company making Pemgarda is gearing up to target new variants as they emerge, Mr. Hering said. But it remains to be seen what will actually happen if and when the virus evolves, said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco.

Still, experts are hopeful. “As long as there’s still this slice of the population that remains highly vulnerable, the whole community has to really be massively cognizant of the virus,” Dr. Mina said.