The Centers for Disease Control and Prevention (CDC) on Thursday determined that vials found at a Pennsylvania facility this week with labels reading "smallpox" did not contain the virus that causes it.
"There is no evidence that the vials contain variola virus, the cause of smallpox," the CDC said in a statement, adding that the agency is "in close contact with state and local health officials, law enforcement, and the World Health Organization about these findings."
On Monday, vials labeled "smallpox" were discovered by a laboratory worker in a freezer at a Merck facility, sparking concern. Due to smallpox's deadly nature, only two labs in the world are authorized to store samples of it.
Staff notified the Department of Homeland Security and the CDC of the incident and vials were sent to the CDC for testing on Thursday, according to the CDC.
"The frozen vials labeled 'Smallpox' were incidentally discovered by a laboratory worker while cleaning out a freezer in a facility that conducts vaccine research in Pennsylvania. CDC, its Administration partners, and law enforcement are investigating the matter and the vials' contents appear intact," the CDC said at the time.
The freezer at the facility was immediately secured and no one was exposed to the vials, the CDC said.