As many as 3,750 medical laboratories, all but 75 of which are in the U.S., have been told by the World Health Organization to destroy a pandemic flu virus sent to them as part of routine testing kits.
The deadly H2N2 virus was mistakenly included in proficiency testing kits sent out by the College of American Pathologists.
This particular virus killed between 1m and 4m people in the 1957-58 global flu pandemic and continued to cause annual epidemics until it vanished in 1968 and people born after 1968 have little or no immunity to it.
If a laboratory worker became ill from the virus it could spark a new pandemic that would travel around the world in a few months.
According to Klaus Stöhr, WHO's influenza co-coordinator, no H2N2 flu outbreak has been reported since the first batch of testing kits was sent to laboratories in October. He says the risk of escape is low and these are laboratories that know what they are doing.
Stöhr says the decision to include H2N2 in proficiency test kits was unwise.
Influenza viruses that are in current circulation would normally be used for proficiency testing, a routine procedure which ensures participating laboratories can identify samples.
Staff at Meridian Bioscience, an Ohio-based company, did not realise the danger when they prepared the samples for the College of American Pathologists, and unwittingly included the H2N2 virus.
The incident does highlight the possible risk of disease from laboratory accidents and raises questions about lab handling of flu viruses and other pathogens. A Canadian laboratory was the first to alert authorities to the mistake when it found H2N2 in a sample not related to the testing kit.