A health worker at a hospital in the U.S. state of Wisconsin was fired after deliberately leaving more than 500 doses of COVID-19 vaccine unrefrigerated for too long, according to reports.

The Aurora Medical Centre in Grafton said it had to discard some 50 vials of Moderna vaccine, each containing 10 doses, after they had been kept out of a refrigerator overnight.

An initial inquiry led Aurora to believe this was a case of “unintended human error,” but the worker admitted on Wednesday to having unfrozen the vaccine on purpose, according to a statement.

Their motives are unknown.

“The individual in question today acknowledged that they intentionally removed the vaccine from refrigeration,” the statement read, adding that the worker was “no longer employed by us.”

The company said it was “more than disappointed” with the worker’s actions that will result in a delay of more than 500 people receiving the vaccine.

The FBI, the federal drug regulator FDA and the Grafton police have all been contacted. (NAN)