Update: Lab testing by the Centers for Disease Control and Prevention (CDC) revealed that the vials marked “smallpox” actually contained vaccinia, the virus used in the smallpox vaccine. They did not find any evidence that the vials contained the variola virus — the virus that causes smallpox, according to a statement the CDC released late on Nov. 18.
While cleaning out a freezer in a Pennsylvania vaccine research facility, a lab worker discovered several frozen vials labeled "smallpox," launching an FBI and CDC investigation, according to recent news reports.
Only two labs in the world — a CDC lab in Atlanta, Georgia, and a Cold War-era bioweapons lab in Russia — can legally hold samples of the deadly virus, Live Science previously reported. In the recent incident, a total of 15 vials — five labeled "smallpox" and 10 labeled "vaccinia" (the virus used in smallpox vaccines) — were discovered in the Merck facility, according to an "official use only" alert sent out to the Department of Homeland Security leadership on Tuesday (Nov. 16), and obtained by Yahoo News (opens in new tab).
"There is no indication that anyone has been exposed to the small number of frozen vials," the CDC said in a statement (opens in new tab) to Yahoo News. "The laboratory worker who discovered the vials was wearing gloves and a face mask." The CDC, its administration partners and law enforcement are now investigating the matter, according to the statement. "We will provide further details as they are available," the CDC said.
Related: The deadliest viruses in history
Smallpox is a highly contagious disease caused by the variola virus; it "was one of the most devastating diseases known to humanity," causing millions of deaths before the world's most successful vaccination campaign led to its eradication in 1980, according to the World Health Organization (opens in new tab).
Smallpox is the only infectious disease to have been eradicated from the planet, with the last known natural case reported in Somalia in 1977.
The investigation led to a lockdown of the research facility that has now been lifted, according to Yahoo News. The alert noted that the CDC would arrive on Wednesday (Nov. 17) to take the vials to another facility for testing, according to Yahoo News.
This isn't the first time that vials of smallpox have been found in a research lab; in 2014, National Institutes of Health employees discovered six vials in a storage room, according to CNN.
Originally published on Live Science.