A coronavirus vaccine cock-up saw 42 people mistakenly injected with the wrong drug, giving them coronavirus antibodies.
The blunder in Boone County, West Virginia, USA, saw dozens of people who were expecting to receive the Moderna jab instead injected with the Regeneron antibody used to treat infections.
Health officials in West Virginia admitted their mistake, adding that everyone given the wrong injection is being contacted.
The West Virginia National Guard is leading the planning and logistics for distributing the vaccine.
Major Gen. James Hoyer, adjutant general of the West Virginia National Guard, said: "The moment that we were notified of what happened, we acted right away to correct it, and we immediately reviewed and strengthened our protocols to enhance our distribution process to prevent this from happening again."
Outgoing US President Donald Trump credits Regeneron with helping him to recover after he was infected with Covid-19 in November.
Dr Clay Marsh, West Virginia's Covid-19 tsar, said: "The products administered are antibodies that fight Covid-19.
"In fact, this product was the same one that was administered to President Trump when he became infected. While this injection is not harmful, it was substituted for the vaccine.
"But this occurrence provides our leadership team an important opportunity to review and improve the safety and process of vaccination for each West Virginian."
The Moderna vaccine became the second jab to be approved by the US Food and Drug Administration in December.
The second was the Pfizer/BioNTech vaccine being used in the UK.
More than 2.7 million doses of the vaccines have been administered in the United States, according to the Centers for Disease Control and Prevention, and more than 12.4 million have been distributed.
Government officials had promised to vaccinate 20 million people by the end of 2020, falling well short of the target.