The Food and Drug Administration has sharply restricted two widely used monoclonal antibody treatments after concluding they are unlikely to work against the Omicron variant, which federal surveillance estimates now accounts for more than 99% of U.S. COVID-19 infections.

The agency on Monday revised the emergency authorizations for Regeneron’s REGEN-COV and Eli Lilly’s bamlanivimab plus etesevimab so the drugs may be used only when a patient is likely to have been infected with, or exposed to, a susceptible variant. Because Omicron is overwhelmingly dominant nationwide, the FDA said the two antibody combinations are not currently authorized for use in any U.S. state or territory.

The decision immediately changes outpatient treatment options at a time when Omicron is producing extraordinary numbers of infections. It also illustrates a central challenge of antibody therapy: unlike broad antiviral medicines, monoclonal antibodies target specific features of the virus and can lose activity when those features mutate.

Two major therapies are effectively removed from use

REGEN-COV, a combination of casirivimab and imdevimab, and Lilly’s bamlanivimab-etesevimab regimen had been deployed extensively during earlier waves. The federal government bought large quantities, distributed doses to states and encouraged treatment of high-risk patients early in infection. Lilly announced a substantial additional federal purchase of its product in a November agreement, underscoring how important antibody therapy had become before Omicron emerged.

Regeneron’s treatment had also been expanded beyond treatment of active disease. In July, the FDA authorized it for certain people exposed to SARS-CoV-2 who were at high risk of progressing to severe illness, a change the company described in an announcement at the time. Those earlier uses were based on variants against which the antibodies retained meaningful neutralizing activity.

Omicron has changed that calculus. Laboratory evidence indicates that the variant’s heavily mutated spike protein substantially reduces the activity of both antibody combinations. The FDA’s archived authorization records now reflect the January 24 revisions, making clear that the legal status of these products depends on whether a susceptible variant is circulating.

Other therapies remain available, but access is uneven

The FDA emphasized that several treatments are expected to retain activity against Omicron. They include sotrovimab, another monoclonal antibody; remdesivir, an intravenous antiviral; Pfizer’s oral drug Paxlovid; and Merck’s oral antiviral molnupiravir. The federal treatment guidance describes the role of monoclonal antibodies and other therapeutics within the broader COVID-19 response.

Those alternatives differ substantially in effectiveness, supply and ease of use. Sotrovimab requires an infusion and has been in limited supply. Remdesivir, long used for hospitalized patients, now has an outpatient regimen that still requires intravenous administration on three consecutive days. Oral antivirals are easier to deliver but must be started quickly after symptoms begin and remain constrained by initial supply.

Paxlovid is particularly important because it combines nirmatrelvir with ritonavir and can be taken at home. The FDA’s December 22 authorization covers high-risk adults and qualifying pediatric patients age 12 and older who weigh at least 40 kilograms, with treatment beginning within five days of symptom onset. The regimen has clinically important drug-interaction considerations, but its oral format offers a way to intervene without arranging an infusion.

Variant biology is becoming a treatment decision

The restriction on REGEN-COV and bamlanivimab-etesevimab means clinicians can no longer treat monoclonal antibodies as interchangeable. The effectiveness of a product now depends directly on which viral variant is circulating, creating a closer link between genomic surveillance and bedside prescribing.

This is not the first time variant resistance has changed an antibody authorization. In April 2021, the FDA revoked authorization for bamlanivimab when used alone because resistant variants had become more common. The earlier decision established the principle that the agency could withdraw or narrow access when the expected benefit of an antibody no longer outweighed its risks under prevailing epidemiologic conditions.

The January action applies the same logic on a much larger scale. With Omicron estimated at more than 99% of U.S. cases by mid-January, the probability that an infected patient has a variant susceptible to the two restricted products is now extremely low. Continuing to administer them would expose patients to infusion risks and consume clinical resources without a reasonable expectation of benefit.

A narrower treatment window during a massive wave

The policy change arrives while hospitals, pharmacies and outpatient clinics are managing intense demand. Omicron generally appears to cause a different pattern of severe disease than earlier variants, particularly among vaccinated people, but the sheer number of infections means large numbers of high-risk patients still need effective early treatment.

For health systems, the operational problem is increasingly one of matching the right patient to a scarce therapy within a short window after diagnosis. Age, immune status, vaccination history, kidney and liver function, medication interactions and access to infusion services can all affect which treatment is appropriate.

The FDA has left open the possibility that REGEN-COV and bamlanivimab-etesevimab could return to use if a susceptible variant becomes common in a particular region. For now, however, federal officials are directing clinicians toward treatments expected to work against Omicron. The change is a reminder that the therapeutic arsenal against COVID-19 is not static: as the virus evolves, the value of individual medicines can change with it.