After an internal investigation, it was first believed to be human error.
The person in question Wednesday admitted that they intentionally removed the vaccine from refrigeration.
That person is no longer employed, Advocate Aurora Health said in a news release.
More than 500 doses of the vaccine had to be thrown out.
The Moderna vaccine can be out of refrigeration for 12 hours
Advocate Aurora Health says it has notified the proper authorities.
More information is expected to be released Thursday.