As nervous American hospitals prepare to respond to a new wave of COVID-19 cases caused by the rapidly spreading omicron mutation, doctors warn of another challenge: The two standard drugs they use to fight infections are unlikely to work against the new strains.

For more than a year, Regeneron and Eli Lilly’s antibody drugs have been the first choice for early COVID-19 treatment, thanks to their ability to prevent serious diseases and keep patients away from hospitals.

But both drugmakers have recently warned that laboratory tests have shown that their therapy is much less effective against omicron, which contains dozens of mutations, making it harder for antibodies to attack the virus. Although these companies say they can quickly develop new antibodies against omicron, they are expected to be launched in at least a few months.

The third antibody from the British drugmaker GlaxoSmithKline seems to be the most suitable against omicron. But GlaxoSmithKline’s drugs are not widespread in the United States, accounting for only a small portion of the millions of doses purchased and distributed by the federal government. US health officials are now rationing scarce drug supplies in states.

“I think there will be a shortage,” said Dr. Jonathan Lee, director of the Harvard/Brigham Virology Laboratory. Due to the reduced effectiveness of Regeneron and Eli Lilly’s drugs, omicron “we can only use one FDA-authorized monoclonal antibody.”

According to the Centers for Disease Control and Prevention, the delta variant still accounts for more than 95% of estimated cases in the United States. But the person in charge of the agency said that omicron is spreading faster than any previous variant and will become the main strain nationwide within a few weeks.

Delivered by injection or infusion, antibodies are laboratory-made versions of human proteins that help the immune system fight viruses and other infections.

According to the two companies, the drugs jointly developed by GlaxoSmithKline and Vir Biotechnology are specially formulated to bind to parts of the virus that are unlikely to mutate. Early studies of laboratory simulation omicron by pharmaceutical manufacturers and external researchers have shown promising results.

The U.S. Department of Health and Human Services said in a statement online that the supply of the drug is “extremely limited, and the additional dose of the product will not be available until the week of January 3.”

After suspending distribution last month to save supplies, HHS is now shipping 55,000 doses of the drug called sotrovimab to the state health department, which arrived as early as Tuesday. It is expected that there will be an increase of 300,000 people in January.

The agency said it is distributing drugs to it based on infections and hospitalizations in each state.

HHS recommends that states retain the drug for the highest-risk patients who are most likely to contract omicron, whether it is based on laboratory tests that can identify the mutation or the level of omicron spread in the local community, which is determined to be 20% or higher.

High-risk patients include the elderly and people with serious health problems such as obesity, heart disease, diabetes, and immune system diseases.

According to federal data, before the suspension of shipments, GlaxoSmithKline’s drugs accounted for approximately 10% of the 1.8 million antibody doses distributed to state health officials from mid-September to late November.

GlaxoSmithKline, headquartered in London, said that under contracts with the United States, Canada, the United Kingdom, Japan and several other countries, it is expected to produce 2 million doses by May. The company is working hard to add more manufacturing capacity next year.

The loss of two leading antibody therapies has brought more attention to a pair of highly anticipated antiviral pills, which are expected to be approved by U.S. regulatory agencies soon.

The Pfizer and Merck drugs will be the first treatment that Americans can use to prevent serious illness at home. Pfizer’s drugs in particular have shown strong effects, reducing the hospitalization rate and mortality of high-risk patients by nearly 90%.

Johns Hopkins University virologist Andrew Pekosz said: “If it is effectively promoted, it will have great potential,” to make up for the lack of antibody therapy. “This is the direct place where these antiviral drugs can minimize the impact of omicron.”

Nevertheless, it is expected that the initial supply of these two drugs will be limited.

The shrinking treatment kit is a painful reminder that even if more than 200 million Americans are fully vaccinated, the virus still prevails in the United States.

Scientists all over the world are racing to understand omicron, including whether it can cause more or less serious diseases, and how it can easily escape the protection of previous infections, vaccinations, and antibody drugs.

“We will definitely see an increase in hospital admissions,” said Dr. James Cutrell of the University of Texas Southwestern Medical Center. “If we lack antibodies, it will definitely cause more patients to be hospitalized.”