Hospitals are scrambling as antibody treatments fail against Omicron.

Hospitals, drug companies and Biden administration officials are racing to address one of the Omicron variant’s biggest threats: Two of the three monoclonal antibody treatments that doctors have depended on to keep Covid-19 patients from becoming seriously ill do not appear to thwart the latest version of the coronavirus.

The one such treatment that is still likely to work against Omicron is now so scarce that many doctors and hospitals have already run through their supplies.

Monoclonal antibodies have become a mainstay of Covid treatment, shown to be highly effective in keeping high-risk patients from being hospitalized. But even as infections surge and Omicron becomes the dominant form of new cases in the United States, some hospitals have begun scaling back the treatments, fearing they have become suddenly useless.

In New York, hospital administrators at NewYork-Presbyterian, N.Y.U. Langone and Mount Sinai all said in recent days that they would stop giving patients the two most commonly used antibody treatments, made by Eli Lilly and Regeneron, according to memos obtained by The Times and officials at the health systems.

“This is a dramatic change just in the last week or so,” said Dr. Daniel Griffin, an infectious disease specialist at Columbia University in New York. “And I think it makes sense.”

Federal health officials plan to assess at the end of this week whether to pause shipments of the Eli Lilly and Regeneron products to individual states, based on how dominant Omicron becomes in different regions of the country, according to a senior administration official who spoke on condition of anonymity.

The one monoclonal antibody treatment that has performed well against Omicron in laboratory experiments is also the most recently authorized: sotrovimab, made by GlaxoSmithKline and Vir Biotechnology and cleared in May.