Clinicians had to discard about 500 doses of the Moderna vaccine Saturday after vials were kept unrefrigerated for too long at Aurora Medical Center in Grafton.
In a statement, Advocate Aurora Health officials said someone removed 50 vials from a refrigerator to access other items and failed to put them back overnight Friday. Each vial contained ten doses of vaccine.
An internal investigation found the failure was an "unintended human error."
"We are clearly disappointed and regret this happened," officials said in a statement.
Clinicians were still able to administer some of the vaccine from the vials within the allowable 12 hour post-refrigeration window but had to discard most of it, according to the statement. Once the vaccine is thawed, it cannot be refrozen.
Advocate Aurora Health has vaccinated about 17,000 of its employees in the last 12 days, officials said.
A total 39,501 doses of COVID-19 vaccine have been administered in Wisconsin, according to state health officials -- mostly Pfizer vaccine and 3,905 doses of Moderna.