Decades-old unaccounted-for vials containing smallpox were discovered in a storage room of a U.S. Food and Drug Administration laboratory in Bethesda, Maryland.
Sixteen sealed and freeze-dried vials containing the virus of the disease, which had been officially declared as eradicated during the 1980s, were found by a government researcher cleaning out the FDA-operated lab at the National Institutes of Health complex, the CDC says.
The vials were discovered during preparations for the lab's move to the FDA's main campus.
"The vials appear to date from the 1950s. Upon discovery, the vials were immediately secured in a CDC-registered select agent containment laboratory in Bethesda," the CDC said in a statement.
The vials are thought to be the first instance of unaccounted-for smallpox turning up in the United States.
"There is no evidence that any of the vials labeled variola (smallpox) has been breached, and onsite biosafety personnel have not identified any infectious exposure risk to lab workers or the public," the CDC said.
After being secured in the Bethesda containment facility, the vials were then flown to CDC headquarters in Atlanta, where the presence of smallpox-virus DNA was confirmed.
Further testing will be necessary to resolve if the material in the vials is still live, the CDC said, noting that such tests could take up to two weeks.
After the testing is complete the samples within the vials will be destroyed, officials said.
Under an international agreement, there are only two places in the world allowed to keep smallpox samples; a secure laboratory at the CDC's Atlanta headquarters and a biotechnology and virology research complex at Novosibirsk in Russia, both overseen by inspections by the United National World Health Organizations.
CDC officials said WHO was notified of the vials' discovery and has been invited to take part in the investigation of how the samples had originally been prepared and their subsequent storage in the FDA laboratory.
WHO will also be allowed to witness their destruction, as has been the case in in other incidents where smallpox samples have been discovered outside of the two official storage repositories.
Smallpox -- which has no cure and kills about a third of people who contract it -- claimed hundreds of millions of victims in the last century until it was successfully eradicated by means of global vaccination efforts.
"It was considered one of the worst things that could happen to a community to have a smallpox outbreak," says Michael Osterholm of the University of Michigan's Center for Infectious Diseases Research and Policy. "It's a disease that's had a major impact on human history."
