LONDON – Thousands of scientists were scrambling yesterday at the urging of global health authorities to destroy vials of a pandemic flu strain sent to labs in 18 countries as part of routine testing.

The rush, urged by the World Health Organization, was sparked by a slim, but real, risk that the samples could spark a global flu epidemic. The vials of virus sent by an American company went to nearly 5,000 labs, mostly in America, officials said.

“The risk is relatively low that a lab worker will get sick, but a large number of labs got it and if someone does get infected, the risk of severe illness is high and this virus has shown to be fully transmissible,” the WHO’s influenza chief, Klaus Stohr, told the Associated Press.

It was not immediately clear why the 1957 pandemic strain, which killed between 1 million and 4 million people, was in the proficiency test kits routinely sent to labs.

It was a decision that Mr. Stohr described as “unwise,” and “unfortunate.”

That particular bug was “an epidemic virus for many years,” Mr. Stohr said from the U.N. health agency’s headquarters in Geneva, Switzerland. “The risk is low but things can go wrong as long as these samples are out there and there are some still out there.”

The 1957 strain has not been included in the flu vaccine since 1968, and anyone born after that date has no immunity to it.

The chief of the influenza branch at the federal Centers for Disease Control and Prevention in Atlanta, Dr. Nancy Cox, said her agency was notified of the situation Friday morning. She also said officials strongly doubt someone deliberately planted the dangerous germ or that this was an act of bioterrorism.

“It wouldn’t be a smart way to start a pandemic to send it to laboratories because we have people well trained in biocontainment,” she said.

The concern over the shipment of pandemic flu virus to thousands of labs renews questions about the safe handling of deadly germs – an issue that led to toughened American rules after anthrax was sent in the mail in 2001, killing five Americans.

Most of the flu samples – 3,747 – were sent starting last year at the request of the College of American Pathologists, which helps labs do proficiency testing. The last shipments were sent out in February.

An official with the pathology college, Dr. Jared Schwartz, said a private company, Meridian Bioscience Inc. of Cincinnati, Ohio, is paid to prepare the samples. The firm was told to pick an influenza A sample and chose from its stockpile the deadly 1957 H2N2 strain.