A US pharmacist is in custody accused of deliberately ruining hundreds of doses of coronavirus vaccine by removing them from refrigeration for two nights.

The Grafton Police Department in Milwaukee said the former Advocate Aurora Health pharmacist was arrested on suspicion of reckless endangerment, adulterating a prescription drug and criminal damage to property.

His motive remains unclear. Police said that detectives believe he knew the spoiled doses would be useless and people who received them would mistakenly think they had been vaccinated when they had not.

Advocate Aurora Health chief medical group officer Jeff Bahr told reporters the pharmacist deliberately removed 57 vials that held hundreds of doses of the Moderna vaccine from refrigeration at a Grafton medical centre overnight on December 24 into Christmas Day, returned them, then left them out again on the night of December 25 into Saturday.

The vials contained enough doses to inoculate 570 people.

A pharmacy technician discovered the vials outside the refrigerator on Saturday morning. Mr Bahr said the pharmacist initially said he had removed the vaccine to access other items in the refrigerator and had inadvertently failed to replace it. However he said the man had since admitted the action was deliberate.

The Moderna vaccine is still viable for 12 hours outside refrigeration, so workers used the vaccine to inoculate 57 people before discarding the rest.

Police said the discarded doses were worth between 8,000 and 11,000 dollars.

Mr Bahr said the doses people received on Saturday are all but useless.

-AAP