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UPDATE: The CDC announced Thursday that the vials did not contain the virus known to cause smallpox but instead contained the virus used in the smallpox vaccine. More details HERE.
Vials labeled with "smallpox" were found at a Merck facility in Montgomery County, Pennsylvania, where vaccine research is conducted, but it is unclear why the vials were there, according to the Centers for Disease Control.
Smallpox is caused by a virus that was eradicated from human transmission in the late 1970s after centuries of epidemics across the globe. It is so deadly that only two laboratories in the world are allowed to have doses of the virus: the CDC main lab in Atlanta and a facility in Russia.
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The CDC said the frozen vials were "incidentally discovered" by a lab worker. The discovery occurred at the Merck Upper Gwenydd facility in North Wales, about 30 miles northwest of Philadelphia, according to a source with knowledge of the ongoing situation. It is not clear exactly when the vials were discovered.
“Merck is in the process of figuring out why it was there," the source told NBC10 on Wednesday.
The CDC said an investigation is underway. Yahoo! News first reported the discovery. Merck did not respond to requests for comment Wednesday morning.
"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," a CDC spokeswoman said in a statement to NBC10. "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."
Smallpox dates back as early as the 6th century, and for centuries spread uncontrolled throughout the world. About 3 in 10 people who contracted the disease died, according to the CDC. It was spread as a virus called variola. A vaccine was invented in 1796, but it wasn't for nearly another 200 years before the last known cases in the late 1970s.
"Following the eradication of smallpox, scientists and public health officials determined there was still a need to perform research using the variola virus. They agreed to reduce the number of laboratories holding stocks of variola virus to only four locations," the CDC says on its website. "By 1984, England and South Africa had either destroyed their stocks or transferred them to other approved labs. There are now only two locations that officially store and handle variola virus under WHO supervision: the Centers for Disease Control and Prevention in Atlanta, Georgia, and the State Research Center of Virology and Biotechnology (VECTOR Institute) in Koltsovo, Russia."