The Centers for Disease Control and Prevention announced Friday that it had identified samples of a “smallpox” virus that were part of samples preserved in liquid nitrogen at an Indiana laboratory in the 1990s.
The CDC’s announcement came five months after the World Health Organization declared the smallpox virus extinct.
However, CDC officials said the samples in question did not contain smallpox virus; rather, they contained virus that did not cause the disease.
“It is expected that the smallpox virus would have escaped if the samples had not been in the freezer,” CDC Director Dr. Brenda Fitzgerald said in a statement.
The CDC said it had been notified of the irregularities in late May and has been working with the U.S. Department of Agriculture, which administered the infectious disease laboratory at the time, to determine who may have been exposed to the pathogens.
“It is possible that persons who are still alive may have been exposed to these pathogens,” the CDC said in a statement.
The CDC said that two other laboratories that handle smallpox are not involved in the investigation.
The National Institutes of Health is performing a thorough analysis of the samples, the CDC said.
The tissue samples were part of a preserved vial containing smallpox virus, which was kept frozen at minus 196 degrees Celsius. When the vial wasn’t used, officials discovered that some of the virus had escaped, the CDC said.