The Food and Drug Administration has granted emergency authorization of a Covid-19 treatment made by the American pharmaceutical company Eli Lilly that was given to Chris Christie, the former New Jersey governor, when he was infected with the coronavirus.

The authorization, announced on Monday, applies only to people newly infected with the virus, and the agency said it should not be used in hospitalized patients. The treatment is approved for people 12 and older who have tested positive and are at risk for developing a severe form of Covid-19 or being hospitalized for the condition. That includes people who are over 65 and obese, the agency said — a key group that early studies have shown can benefit the most from the treatment.

Eli Lilly said that its treatment, called bamlanivimab, should be administered as soon as possible after a positive test, and within 10 days of developing symptoms.

“It’s a great day for science and medicine — sort of a feat of what’s possible,” said Dr. Daniel M. Skovronsky, the chief scientific officer of Eli Lilly. The company and its collaborators, including the National Institutes of Health, he said, were able “to create a new drug, manufacture it, test it in clinical trials, and get it authorized for use in just seven months.”

In October, the company announced that it had reached a $375 million deal to sell 300,000 doses of the treatment to the U.S. government. Eli Lilly said Monday that it would begin shipping the treatment immediately to AmerisourceBergen, a national distributor, which will then distribute it on behalf of the federal government.

The treatment consists of a single powerful antibody that is believed to keep the infection in check and has been shown in early studies to reduce emergency room visits and hospitalizations in patients who get the drug early in the course of their disease. It is similar to the treatment that President Trump received, made by the American biotech company Regeneron, which is a cocktail of two antibodies. Regeneron has also applied for emergency authorization.

Mr. Christie disclosed this fall that he had received the Lilly treatment on an emergency basis after he was infected with the virus.

Eli Lilly’s authorization raised immediate questions about who would get access to the treatments, which must be infused in a clinic or hospital. The company has said it expects to have enough to treat one million people by the end of the year. That means, even in the best-case scenario, there won’t be nearly enough to curb a virus that is now infecting an average of more than 116,000 people a day in the United States.

“It’s kind of the best times for these therapies to enter because they can have an impact,” said Dr. Walid Gellad, who leads the Center for Pharmaceutical Policy and Prescribing at the University of Pittsburgh. “It’s also the worst time because we don’t have enough doses, and it’s going to add to the backlog of testing.”

More than 130,500 new cases were announced in the United States on Monday, the second-highest total of the pandemic, and the sixth day in a row the country exceeded 100,000 cases in a single day. Twelve states and the territory of Guam set single-day records for new cases.