London ? Thousands of scientists were scrambling Tuesday 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 a U.S. company went to nearly 5,000 labs, mostly in the United States, 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,” 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 Stohr described as “unwise,” and “unfortunate.”

That particular bug was “an epidemic virus for many years,” 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.