The World Health Organisation is urging thousands of laboratories around the world to destroy samples inadvertently sent to them of the Asian flu virus that killed up to four million people in 1957-58.
Scientists feared an accident could trigger a new global outbreak.
The virus, known as H2N2, killed about four million people worldwide during the Asian influenza pandemic of 1957-58 before disappearing in 1968.
"Therefore, persons born after 1968 are expected to have no or only limited immunity to H2N2," which is not contained in current trivalent influenza vaccines, the WHO warned Tuesday in a statement.
The US government on April 8 asked the College of American Pathologists (CAP) to instruct 3,747 laboratories in 18 countries that received samples containing the H2N2 virus to destroy them, the WHO said.
The H2N2 samples were shipped to the laboratories -- most of which are in the United States with only 75 located in Canada and 16 other countries -- by Meridian Bioscience Inc. of Cincinnati, Ohio as part of "routine quality-control certification conducted" by the College of American Pathologists.
Stohr called the private company's decision "unwise and unfortunate," while Robert Webster, a flu expert at St. Jude's Children's Research Hospital in Memphis, Tennessee, called the incident "a terrible, terrible mistake."