The Food and Drug Administration has withdrawn emergency use authorization for AstraZeneca’s Covid-19 antibody cocktail, Evusheld, as the treatment is not expected to neutralize omicron’s currently dominant XBB.1.5 subvariant.

The FDA had limited the use of Evusheld before this month for the same reason and the agency’s announcement on Thursday sent US shares in London-based AstraZeneca tumbling 1.31% to $65.75.

Omicron’s XBB.1.5 subvariant has been spreading rapidly in the country since December and accounted for almost half of all Covid cases in the US. In the past weekaccording to government data.

FDA decision resonates with concerns raised by the European Union health regulator on the efficacy of monoclonal antibodies such as Evusheld against new variants.

AstraZeneca said it was told the agency will decide on reinstating Evusheld’s authorization if the prevalence of resistant variants in the United States declines to 90% or below on a sustained basis.

The drugmaker plans to continue sharing relevant data with the FDA and other health authorities regarding Evusheld and SARS-CoV-2 variants.

Monoclonal antibodies work by binding to the spike protein on the surface of the SARS-CoV-2 virus, but the virus has evolved, causing changes to this protein and affecting how antibodies work against them.

In NovemberThe US health regulator has also withdrawn the emergency use authorization for Eli Lilly’s Covid drug, bebtelovimab, citing similar concerns.

Evusheld was first authorized in late 2021 by the FDA to prevent covid infections in people with weak immune systems or a history of serious side effects from coronavirus vaccines.

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