Vials labeled as smallpox were discovered at a Pennsylvania lab, the Centers for Disease Control and Prevention said Wednesday.
The frozen vials "were incidentally discovered by a laboratory worker while cleaning out a freezer in a facility that conducts vaccine research in Pennsylvania," a CDC spokesperson said in a statement to NBC News.
The contents appeared to be intact, and the CDC is partnering with law enforcement to investigate, the agency said.
“The laboratory worker who discovered the vials was wearing gloves and a face mask. There is no indication that anyone has been exposed to the small number of frozen vials. We will provide further details as they are available," the spokesperson said.
Smallpox was eradicated in the 1970s, but vaccines still exist in case of an outbreak, according to the CDC. Once a person develops a smallpox rash, the vaccine cannot protect them. About 3 in 10 people with smallpox die.
NBC Philadelphia reported that the vials were found at a Merck facility in Montgomery County, about 30 miles northwest of Philadelphia. Only two facilities in the world are supposed to have doses of the virus: CDC's Atlanta headquarters and a Russian lab.
Merck did not respond to a request for comment Wednesday. The Department of Health and Human Services also did not respond to a request for comment.