The Centers for Disease Control and Prevention on Wednesday released the results of its internal investigation of the December 22 incident in Atlanta.
The mistake occurred during a study of how long Ebola-infected animals remain infectious. Lab technicians were working with cheek swabs from some guinea pigs that had survived exposure to the virus and some that had died, CDC officials said.
The second lab, in the same CDC building as the first, was supposed to get samples that had been treated with a solution to kill the virus. But two trays of tubes were mixed up, and it received untreated samples instead.
A technician in the second lab worked with them before the mix-up was discovered.
The technician was considered at risk and was monitored for 21 days for symptoms of Ebola — a period that ended January 12. No symptoms appeared.
CDC officials now think it's unlikely the samples were dangerous. No live virus was found in other untreated samples taken before and after the ones that were sent between labs, said Michael Shaw, the CDC's senior adviser for laboratory science.
The report faults inadequate safeguards, including lack of a written study plan and insufficient monitoring for potential errors like this.
A CDC spokesman declined to say whether any employees are being fired or re-assigned as a result of the incident, saying the agency as a policy doesn't release details on disciplinary actions of personnel.
The CDC is currently recruiting for an administrator to oversee lab safety.