Study results strengthen the case for using Regeneron’s antibody cocktail in high-risk Covid patients.

A monoclonal antibody treatment developed by the drug maker Regeneron sharply cut the risk of hospitalization and death when given to high-risk Covid-19 patients in a large clinical trial, the company announced on Tuesday.

The results are the latest in a growing flurry of evidence that the infused drugs, meant to mimic the antibodies that the immune system generates naturally in fighting the coronavirus, can help infected patients avoid the worst outcomes if given early.

Regeneron’s treatment, a cocktail of two antibody drugs, was given last fall to President Donald J. Trump shortly after he got sick with Covid-19 and is now one of three such therapies available in the United States.

The new results come from a Phase 3 trial that enrolled more than 4,500 patients beginning in late September, around the time virus cases began to climb dangerously in the United States. The study found that patients who got the infused treatment within 10 days of developing symptoms or testing positive had a roughly 70 percent reduced risk of being hospitalized or dying compared with patients who were infused with a placebo.

“I think these are exciting data,” said Dr. Rajesh Gandhi, an infectious diseases physician at Massachusetts General Hospital who was not involved in the study.

Even as vaccinations speed up, antibody treatments are expected to be helpful for high-risk people who still get sick for many months at least, and longer still if the virus can’t be wiped out. While there are signs that emerging virus variants may in some cases make antibodies less potent, Regeneron’s cocktail has not shown such vulnerability in laboratory tests.

In the new findings, Regeneron’s treatment worked equally well when given at half the dosing at which it was authorized. Regeneron said that it planned to request that the Food and Drug Administration allow the treatment to be given at that reduced strength.

Such a change would bring several advantages: While the cocktail is safe, getting it at a lower dose reduces the odds of side effects, such as an infusion reaction.

It would also allow Regeneron to increase the supply it can provide the United States. The company said that it had expected to supply the country with about 750,000 doses at the originally authorized higher strength by the end of June. If the lower strength is authorized, the company expects to provide about 1.25 million doses by then.

The antibody treatments from Regeneron and the drug maker Eli Lilly, which makes the other two such drugs authorized in the United States, were expected to be in high demand and to serve as a bridge in fighting the pandemic before vaccinations ramped up. Instead, they ended up sitting on refrigerator shelves in many places even during recent surges.

Many patients and their doctors did not know to ask for them or where to find them. Overwhelmed hospitals lacked the bandwidth to prioritize giving out the treatments. And some doctors were unconvinced by the relatively weak evidence available last fall supporting their use.

That picture is gradually shifting, thanks to improved logistics and more awareness. And more solid evidence, like the new data from Regeneron, also appears to be helping the drugs get used more widely. “As the data get stronger and stronger, I would expect that use will increase,” Dr. Gandhi said.