Forty-four people received an antibody treatment instead of Moderna’s coronavirus vaccine in an error Wednesday at a vaccination clinic in Boone County, W.Va.

The West Virginia National Guard, which announced the error Thursday, said medical experts advising the state’s covid-19 response “do not believe there is any risk of harm” to the people. The Guard’s statement indicated 42 residents were affected, though county officials said the number was actually 44.

These people would now receive priority access to the vaccine, state and county officials vowed. They said the drug, especially because it was administered as a shot, would not pose obstacles to inoculation.

The antibody treatment, developed by Regeneron Pharmaceuticals, is designed to prevent people infected with covid-19, the disease caused by the coronavirus, from developing severe illness. It was given to President Trump when he was stricken by the virus in October and granted emergency use authorization by the Food and Drug Administration in November.

State officials highlighted its use by the president in stressing that the mix-up would not have dangerous consequences.

“In fact, this product was the same one that was administered to President Trump when he became infected,” said Clay Marsh, coordinator of West Virginia’s coronavirus task force. “While this injection is not harmful, it was substituted for the vaccine.”

Officials in Boone County said the problem arose when the county health department dispatched two people to retrieve vaccine supplies from the Charleston Area Medical Center and returned with a small white box bearing few markings.

At the regional hospital, the county workers “signed a chain of custody form that said, ‘Moderna covid-19 vaccine,’ ” said Michael Mayhorn, director of emergency management in Boone County, which is in southwestern West Virginia and claims to be the birthplace of the American coal industry. “The packaging, the vials — there was nothing on them that said anything different.”

The health department began administering the antibody treatment as an injection to adults 80 and over, said Philip A. Galapon, a county health officer, before receiving a call from the National Guard. Other locations that had received shipments of the antibody treatment rather than the vaccine noticed the mix-up, he said, and alerted the state.

The mistake is highly unusual because Regeneron’s drug, a cocktail of two monoclonal antibodies, is given by infusion, rather than as a shot. Much of the medication has gone unused because of logistical complexities involved in its delivery. Hospitals and health departments battling a surge of coronavirus infections have not had the resources to plan for the drug’s administration or to muster the workforce needed to monitor patients for rare allergic reactions.

Now, the same medical providers are adding vaccination to their portfolio.

West Virginia’s vaccination effort is unique in relying heavily on its National Guard, though members are not involved in actual inoculations, just transport and logistics, said a spokeswoman, Holli Nelson. The unit’s leader, James A. Hoyer, is heading the campaign, which has already successfully brought shots to every long-term care facility in the state, officials say.

But the episode highlights missteps happening across the country, as the most ambitious vaccination campaign in U.S. history gets underway. Just 23 percent of the 12.4 million vaccine doses distributed as of Wednesday have been put into people’s arms, according to the Centers for Disease Control and Prevention. Trump administration officials insist these figures lag behind the actual number of vaccinations and also say the pace will begin to accelerate next week.

“We need to remember that these are new vaccines on new platforms with slightly complex requirements for storage, handling and administration,” Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said during a Wednesday briefing.