How monoclonal antibodies work against omicron variant

With omicron's explosive spread threatening to outpace current COVID-19 treatments, the race is on to find new options that will stand up to the variant.

The first oral antiviral treatments for the virus were authorized by the Food and Drug Administration this week: Pfizer's Paxlovid, and Merck's Molnupiravir, which have proved effective against severe illness and death from COVID, and are expected to hold up against omicron. The news lent a glimmer of hope amid the latest surge -- but supply is expected to remain limited for months as production ramps up.

Meanwhile, omicron's high transmission rate is squeezing what was already a finite arsenal of COVID medicines. Of the currently authorized monoclonal antibody therapies -- which have become a primary means to keep patients out of the hospital -- two of the main ones bought in bulk by the U.S. government have not been successful against the omicron variant.

Because of those omicron efficacy concerns, federal health officials announced Thursday they would pause shipments of both those treatments, from Eli Lilly and Regeneron.

And a third option, one so far expected to remain effective against omicron, is in scant supply.

Federal health officials are now moving quickly to stockpile the drug, called sotrovimab, from Vir Biotechnology and GlaxoSmithKline.

Dr. Anthony Fauci, the nation's top infectious disease expert, said the low supply of sotrovimab is "something we need to worry about" in a private call this week between the White House COVID response team and the nation's governors obtained by ABC News.

The Biden administration is aiming to double its sotrovimab supply to more than 1 million treatment courses, making 300,000 doses available in January.

So far about 55,000 doses of sotrovimab have been allocated to states, with federal health officials promising shipments would arrive as soon as this week. But as the omicron variant rapidly advances -- nearly three-quarters of all COVID cases are now omicron -- health officials are bracing for a winter surge while potentially lacking ample defense from treatments that hold up against the new variant.

Marcus Plescia, chief medical officer of the Association of State and Territorial Health Officials, told ABC News that state leaders gearing up for omicron face a multipronged challenge: what was a "compelling tool" against the virus is now compromised, and what now works is still in limited supply. In addition, with hospital staffing shortages, there are looming concerns that manpower to administer the treatment will also be scarce.

"We can't just hope it's delta and give the older therapies," he said. "Even if all of these monoclonal treatments still worked against all the variants, and we had an abundant supply, I'd worry we would get to a place where we just didn't have the capacity to administer them."

Evusheld, the pre-exposure monoclonal treatment from AstraZeneca authorized earlier this month and expected to hold up against omicron, is another treatment that could help out health officials. It can be given to a small subset of people for preventative use against the virus, such as those where the vaccines are not effective because of compromised immune systems.

"We're in a very tight situation now where the virus is increasing faster than our access to the drugs that can treat it," said Dr. Andrew Pavia, a fellow with the Infectious Diseases Society of America and a member of the National Institutes of Health Covid treatment guidelines panel, told ABC News. "Once we have an adequate supply of sotrovimab, of Evusheld and of Paxlovid, we'll have some pretty good tools for fighting omicron. But the virus is going to wash over the country before we have an adequate supply. So I'm worried January is going to be ugly."

A spokeswoman for GSK told ABC News the company is "working with urgency and exploring options to expand our supply capacity in 2022," including securing additional batches of the ingredients they'll need to amp up sotrovimab manufacturing, and pushing up their next year supply plans to make more available sooner.

"The challenge is that the supplies [of sotrovimab] are still very limited," Pavia said. "It's going to require some very careful prioritization of who needs the drug the most."

It's not the first time new variants have hindered mainstay COVID treatments: for several months earlier this year federal health officials paused distribution of Eli Lilly's monoclonal antibody treatment when it was shown to be ineffective against the gamma and beta variants. The Food and Drug Administration recommended health care providers use alternative authorized antibodies -- like Regeneron and GSK -- which had shown to hold up against the main variants of concern.

The government spent billions of dollars purchasing Regeneron and Eli Lilly's monoclonal antibody cocktail to ensure there would be enough supply; GSK's had not been as widely purchased and distributed. When delta became the dominant variant of concern and all three antibody therapies were effective against it, Eli Lilly's authorization was renewed.

Health officials have increasingly turned to these therapies as breakthrough cases have ticked up and vaccination rates have not ticked up enough. These antibodies are synthetic versions of the body's natural line of defense, meant for COVID patients early on in their infections and who are at high risk of getting even sicker in order to help keep them out of the hospital.

The currently authorized COVID-19 vaccines and their booster doses have still shown to be highly effective at preventing severe illness and death.

Some hospitals, like Mount Sinai in New York, have already put a pause on using Regeneron and Eli Lilly's treatments, given they are not effective against the variant. Some jurisdictions, however, are still clamoring for more supply amid the new crush of cases.

Miami-Dade County exhausted its supply of Regeneron's treatment this week, shutting down its infusion site for a day while more was secured, a spokesperson for the mayor's office told ABC News. They now have enough to last until Monday.

"What's emerging is a realization that this virus is going to be with us for a while, and it's going to be challenging," Plescia said. "We're just going to have to get through this as best we can and find new tools to meet that challenge."

ABC News' Eric M. Strauss and Cheyenne Haslett contributed to this report.