U.S. health regulators said Monday that COVID-19 antibody drugs from Regeneron and Eli Lilly should no longer be used because they are unlikely to fight the omicron variant that is now nearly all U.S. infections.

The U.S. Food and Drug Administration said it was revoking emergency authorizations for the two drugs, which are purchased by the federal government and given to millions of Americans with COVID-19. If the drugs prove effective against future variants, the FDA said it could reauthorize their use.

The regulatory move was expected, as both drugmakers said infusion drugs were less able to target omicron because of the omicron mutation. Still, the federal action could spark resistance from some Republican governors, who continue to push the drugs against the advice of health experts.

Omicron’s resistance to two leading monoclonal antibody drugs has upended the COVID-19 treatment regimen in recent weeks.

Doctors have alternative treatments to combat early COVID-19 cases, including two new antivirals from Pfizer and Merck, but both are in short supply. A still-effective antibody drug from GlaxoSmithKline is also in short supply.

These drugs are laboratory-made virus-blocking antibodies. They are designed to stop serious illness and death by delivering concentrated doses of one or two antibodies early in infection. Then-President Donald Trump received Regeneron’s antibody combination after testing positive for the 2020 coronavirus.

The U.S. government temporarily halted distribution of both drugs in late December as omicron is racing to become the lead variant across the country. But officials resumed distribution after complaints from Republican governors, including Florida’s Ron DeSantis, who claimed the drugs continued to help some omicron patients.

DeSantis has vigorously promoted antibody drugs as a signature part of his administration’s response to COVID-19, setting up infusion sites and praising them in news conferences, while opposing vaccine mandates and other public health measures. Texas Governor Greg Abbott also launched state-funded infusion sites.

Not a Modern Healthcare subscriber? Register today.

These drugs are not a substitute for vaccinations and are usually reserved for the most vulnerable, including the elderly, transplant recipients and people with conditions such as heart disease and diabetes.

The U.S. government has shipped enough doses of the two antibodies to treat more than 300,000 patients since early January.

Both Regeneron and Lilly previously announced that they were developing new antibodies against omicron.

The move comes days after regulators expanded the use of remdesivir, the first drug approved for COVID-19, to treat more patients.

On Friday, the FDA expanded the approval of the antiviral drug to include adults and children with early-stage COVID-19 who are at high risk for hospitalization. Remdesivir was previously limited to hospitalized patients.

An influential federal panel of experts has recommended the infused drug to avoid hospitalization. The same guidelines from the NIH panel advise against continued use of Eli Lilly and Regeneron’s antibody drugs because they are less effective against omicron.

Still, many hospitals will face challenges in scaling up remdesivir treatment. When used in non-hospitalized patients, the drug requires three consecutive intravenous infusions over three days. For many hospitals with excess capacity facing staff shortages, this time-consuming process will not be an option.

The FDA’s decision was based on a study of 560 patients that showed that giving remdesivir within seven days of onset of symptoms reduced hospitalization rates by nearly 90 percent. The research predates the omicron variant, but like other antivirals, remdesivir is expected to maintain its performance against the latest variant.