The state's National Guard said 42 people received the treatment, which is given intravenously, at a vaccination clinic staffed by the Boone County Health Department. The National Guard said it learned of the error on Wednesday.
Everyone who received the antibody treatment instead of the vaccine, which is given via a shot in the arm, have been contacted, Julie Miller, an administrator for the Boone County Health Department, told CNBC by email. She added that "we do not believe there is any risk of harm."
Regeneron's monoclonal antibody treatment, which needs to be administered through an IV drip, is seen as a promising treatment for Covid-19 — especially when administered early on in the course of infection. But the mix-up in West Virginia is just one example of confusion in the rush to distribute the vaccine to tens of millions of people. The rollout has been slower than expected and marked by logistical challenges.
"It has been determined that this was an isolated incident," Miller said. "All of the affected individuals will be offered the COVID-19 vaccine today."
She said the health department will work closely with the state National Guard and the Department of Health and Human Resources to review its policies and procedures.
Miller did not provide any details on what caused the mix-up.
Representatives for the West Virginia National Guard and the West Virginia governor's office did not return CNBC's request for further comment on how the mistake occurred.
Maj. Gen. James Hoyer, adjutant general of the West Virginia National Guard, said in a statement that his troops "acted right away" to correct the error as soon as they found out what happened. "We immediately reviewed and strengthened our protocols to enhance our distribution process to prevent this from happening again," he said in a statement.
He added that the state will continue to ramp up distribution of the vaccine "to save more and more lives every single day."
Dr. Clay Marsh, the state's Covid-19 Czar, noted in a statement that the Regeneron treatment mistakenly administered instead of the vaccine is the same product "that was administered to President Trump when he became infected."
"While this injection is not harmful, it was substituted for the vaccine," he said. "But this occurrence provides our leadership team an important opportunity to review and improve the safety and process of vaccination for each West Virginian."