The Biden administration buys 100,000 doses of a combination antibody treatment for high-risk Covid-19 patients.

The federal government has agreed to buy 100,000 doses of a recently authorized Covid-19 treatment from Eli Lilly, increasing the supply of such drugs for patients who are high risk of becoming seriously ill but are not yet hospitalized.

Under the deal, announced on Friday, the government will pay $210 million and Eli Lilly will ship out the doses by the end of March. The government has the option to buy 1.1 million more doses of the treatment through November, but how many of those doses ultimately get ordered will depend in part on the course of the pandemic in the United States.

The treatment is a cocktail of monoclonal antibodies combining the Eli Lilly drug known as bamlanivimab — which was authorized last November and is in use for high-risk Covid-19 patients — with a second drug known as etesevimab. The combination received emergency authorization earlier this month from the Food and Drug Administration. Both drugs consist of artificial copies of the antibodies that are naturally generated when a person’s immune system fights off an infection.

The U.S. government previously agreed to buy nearly 1.5 million doses of bamlanivimab alone. Eli Lilly has delivered more than 1 million doses already, with the remainder to be delivered by the end of March. More than 660,000 doses of bamlanivimab have been shipped out to states and other jurisdictions.

Eli Lilly’s new combination therapy could offer an advantage over bamlanivimab alone if worrisome coronavirus variants — particularly B.1.351, the one first identified in South Africa — take off in the United States. While bamlanivimab alone was found in a lab study to be powerless against the B.1.351 variant, preliminary data suggest that the combination therapy may be better able to fight off variants. That’s because so-called escape mutations in the variants that may enable them to avoid one antibody may not work against the second.

Another monoclonal antibody cocktail, made by Regeneron, is also authorized in the United States. Nearly 100,000 doses of that therapy have been shipped out.

Antibody treatments got a publicity boost last fall when they were given to Donald J. Trump when he was infected in the last months of his presidency, and to other high-profile Republicans, but they were surprisingly underused in many places in their first months of availability. Overwhelmed hospitals did not prioritize the treatments, which are cumbersome and must be given via intravenous infusions. Many patients and doctors did not know to ask for them or how to find them.

In December, the federal government’s early data collected from hospitals suggested that they had given only about 20 percent of their supply to patients. But that picture is changing. Eli Lilly has seen usage of bamlanivimab alone rise to around 40 percent nationwide, with uptake much higher in some places, Janelle Sabo, who leads Eli Lilly’s work on Covid-19 antibodies, said in an interview earlier this week.