This article has been updated with the results of the C.D.C.’s investigation.
After an investigation into vials labeled “smallpox” found at a laboratory in Pennsylvania, the Centers for Disease Control and Prevention said late Thursday that it had found no evidence the vials contained the variola virus, the cause of the disease.
The vials “were incidentally discovered by a laboratory worker while cleaning out a freezer in a facility that conducts vaccine research in Pennsylvania,” Belsie González, a spokeswoman for the C.D.C., said in an email early Thursday.
Late in the day, the agency announced that laboratory tests showed that five vials labeled “smallpox” contained vaccinia, the virus used in the smallpox vaccine.
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“There is no evidence that the vials contain variola virus, the cause of smallpox,” the C.D.C. said in a statement on Thursday.
The health agency added that it was “in close contact” with state and local health officials, law enforcement and the World Health Organization about the findings. The vials had been sent to the C.D.C. for testing on Thursday after they were found.
The C.D.C. did not say where the vials were discovered or how many there were except that they were found in a facility that conducts vaccine research in Pennsylvania.
Mark O’Neill, a press secretary for the Pennsylvania Department of Health, said there were a “small number of vials” found at a Merck facility in Montgomery County, outside Philadelphia.
Merck did not respond to a request for comment on Thursday. The F.B.I. referred inquiries to the C.D.C.
“The Pennsylvania Department of Health would like to stress that there has been no known threat to public health and safety,” Mr. O’Neill said.
Citing a notification from the state’s department of health, Kelly Cofrancisco, a spokeswoman for the Montgomery County Office of Communications, said there was a total of “15 questionable vials” with five labeled “smallpox” and 10 as “vaccinia.”
Smallpox, an infectious disease caused by the variola virus, caused devastating outbreaks for centuries, with about three of every 10 cases proving fatal, according to the C.D.C.
Symptoms include a very high fever and a blistering, progressive skin rash.
The virus claimed the lives of 300 million people in the 20th century, according to the W.H.O.
In the event of an outbreak, the C.D.C. said, “there is enough smallpox vaccine to vaccinate every person in the United States.”
The agency said that the last natural outbreak of smallpox in the United States was in 1949. The last known natural case was in Somalia in 1977, according to the W.H.O.
The W.H.O. said that there were two authorized repositories of variola virus stocks, with the C.D.C. in Atlanta and at a research center in Russia. In July 2014, six glass vials that contained the smallpox virus were found in a storeroom in a government laboratory outside Washington. At the time, the C.D.C. said there was no indication that lab workers or the public had been exposed to the contents.
The C.D.C. said that smallpox research in the United States focused on the development of vaccines, drugs and diagnostic tests to protect people against smallpox in the event that it is used for bioterrorism.
Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital in New York, said that smallpox can be lethal “even after it is freeze-dried.”
He said that because of its highly infectious nature, “the virus itself must be kept cold.” At room temperature after many years, he said, it was “unlikely that the virus would retain any ability to infect people.”
Dr. Glatter added that there had been an ongoing debate about whether governments should retain viral samples or eliminate all known copies of the virus.