Federal investigators are probing how vials of smallpox made their way into a storage room at a Food and Drug Administration lab near the US capital, health authorities said Tuesday.
Smallpox is a highly contagious and sometimes fatal disease that is estimated to have killed some 300 million people in the 20th century alone.
Though there is no treatment for smallpox, it has been eradicated after a worldwide vaccination program. The last US case was in 1949; the last global case was in 1977 in Somalia.
The vials were labeled "variola," another name for smallpox, and appear to date from the 1950s, the US Centers for Disease Control and Prevention said in a statement.
They were found in an unused portion of a storeroom in an FDA laboratory, located on the National Institutes of Health campus in Bethesda, Maryland.
There is no evidence that the vials had been opened, and "onsite biosafety personnel have not identified any infectious exposure risk to lab workers or the public," the CDC said.
The vials have been moved to a high-security lab at the CDC headquarters in Atlanta, Georgia.
Initial tests came back positive for smallpox, and further testing will be done to determine if it is viable, or able to grow in tissue culture.
"This testing could take up to two weeks. After completion of this testing, the samples will be destroyed," the CDC said.
- Will be destroyed -
"If viable smallpox is present, the World Health Organization will be invited to witness the destruction of these smallpox materials, as has been the precedent for other cases where smallpox samples have been found outside of the two official repositories."
According to international agreements, only two places in the world are authorized to keep samples of smallpox: the CDC in Atlanta and the State Research Centre of Virology and Biotechnology (VECTOR) in Novosibirsk, Russia.
Global stockpiles of smallpox still exist so that researchers can study them for vaccines and potential drug treatments in case another outbreak were to occur.
The CDC has warned of "heightened concern that the variola virus might be used as an agent of bioterrorism," particularly in the aftermath of the September 11, 2001 attacks.
"For this reason, the US government is taking precautions for dealing with a smallpox outbreak," the CDC website has said on a page devoted to general information about smallpox.
In its statement Wednesday, the CDC said it was notified by the NIH of the discovery on July 1, when workers were preparing to move the FDA lab from the NIH to the FDA's main campus.
The CDC Division of Select Agents and Toxins (DSAT) and the FBI are investigating how the vials got there.
A White House official said that officials in the administration, including in the National Security Council, had been briefed on the incident.
Discovery of the smallpox vials came just weeks after the CDC in Atlanta announced that 80 or more workers may have been accidentally exposed to anthrax.
The "unintentional exposure" occurred at a high-security lab "after established safety practices were not followed," the CDC said on June 19.
Anthrax gained notoriety after a spate of US mail attacks in 2001 killed five of 22 people infected.