The U.S. government on April 5 immediately halted all shipments of a COVID-19 treatment to states and ordered health care providers not to use the drug.

The actions were taken because of the increasing prevalence of BA.2, a subvariant of Omicron, which is a variant of the CCP virus, the virus causes COVID-19.

Data so far indicate that the monoclonal antibody called sotrovimab doesn’t work against BA.2, according to the U.S. Food and Drug Administration (FDA).

Regulators initially told doctors in eight states on March 27 to stop administering sotrovimab to COVID-19 patients. Several days later, the FDA expanded the action, which essentially revokes the emergency use authorization granted in 2021, to 14 additional states.

With the April 5 move, the monoclonal therapy “is no longer authorized to treat COVID-19 in any U.S. region due to increases in the proportion of COVID-19 cases caused by the Omicron BA.2 subvariant,” the FDA said in a statement.

The regulator is relying on estimates from the Centers for Disease Control and Prevention, which pegs BA.2 as causing the majority of COVID-19 cases in all regions of the country.

According to pseudovirus testing performed by the FDA, sotrovimab worked about 16 times less well against BA.2. As a result, it’s unlikely that the authorized dose is effective against the subvariant, the FDA told health care providers in a recently updated fact sheet.

After the FDA’s update, the Department of Health and Human Services announced it was stopping all shipments of sotrovimab.

Some 16,600 doses were allocated this week, down from over 50,000 per week in the weeks before BA.2 was pegged as causing a majority of cases in some regions.

Providers should turn to the remdesivir or paxlovid treatments from Gilead Sciences and Pfizer, respectively, according to the COVID-19 Treatment Guidelines Panel.

In situations where neither are available, providers should use bebtelovimab, a monoclonal from Eli Lilly, or Merck’s molnupiravir, the panel said.

GlaxoSmithKline and Vir Biotechnology, which make sotrovimab, didn’t respond to requests for comment.

The companies have said data they generated suggested the drug retained neutralizing activity against BA.2. After the FDA ordered providers in some states to stop using the drug, the companies said they were preparing a package of data “in support of a higher dose of sotrovimab for the Omicron BA.2 subvariant and will be sharing these data with regulatory and health authorities around the world for discussion.”