"It's very important these samples be destroyed very, very rapidly," Dr. Klaus Stohr of the World Health Organization told CBS Radio News.
Nearly 5,000 labs in 18 countries, received the virus from a U.S. company that supplies kits used for quality control tests.
"The risk is low and we've taken appropriate action," said Dr. Nancy Cox, chief of the influenza branch at the Centers for Disease Control and Prevention in Atlanta.
Stohr, her counterpart at WHO, agreed but said, "If someone does get infected, the risk of severe illness is high, and this virus has shown to be fully transmissible."
The mistaken distribution actually occurred in October, reports CBS News Health Correspondent Dr. Emily Senay.
"I think this is a theoretical risk. There is no outbreak at the moment," Senay told Early Show co-anchor René Sylver. "The concern is that it would escape from one of these laboratories, possibly infect one of the workers, and go from there. But this has not happened yet, and these laboratories ... are accustomed to working with virulent pathogens."
The germ, the 1957 H2N2 "Asian flu" strain, killed between 1 million and 4 million people, 70,000 in the U.S. alone. It has not been included in flu vaccines since 1968, and anyone born after that date has little or no immunity to it.
Still, the decision to send out the strain was described by Stohr as "unwise" and "unfortunate."
The CDC learned Friday that test kits prepared by Meridian Bioscience Inc. of Cincinnati contained the virus. The company makes kits for at least four groups that help labs do proficiency testing, which involves identifying viruses to check a lab's quality controls or to acquire certification.
"These vials were sent to these laboratories as sort of a pop quiz for their certification to make sure that they were capable of accurately identifying different flu strains," said Senay. "It's normal business, business as usual.
"Unfortunately, it seems there was some sort of clerical error and when they chose the flu strains to go into these vials as part of these quizzes so they could be certified, they included a strain that shouldn't have been in there," Senay said.
The largest of the four groups who distributed the kits, the College of American Pathologists, said it had sent 3,747 kits to various labs starting last year and ending in February.
Dr. Jared Schwartz, an official with the pathology college, said Meridian was told to pick an influenza sample and chose from its stockpile the deadly 1957 strain, which it had received from a "germ library" in 2000.
Other test kit providers also used the strain. Schwartz identified them as Medical Lab Evaluators, the American Association of Bioanalysts and the American Association of Family Practitioners.
Some of the labs outside the United States have already incinerated their samples, Stohr said, and WHO hoped the rest of the vials would be destroyed by Friday.
The kits contain blind samples that labs must correctly identify to pass the test. The influenza virus included in the kits typically is one that is currently circulating or has recently circulated.
A Canadian laboratory detected the 1957 pandemic strain on March 26 in a sample that was later traced to a test kit.
The WHO notified health authorities in countries that received the kits and recommended that all samples be destroyed. The College of American Pathologists asked labs to incinerate the samples immediately and confirm their actions in writing.
The virus' presence in thousands of labs focused fresh attention on the safe handling of deadly germs — an issue that led to toughened U.S. rules after anthrax was sent in the mail in 2001, killing five Americans.
Cox said officials strongly doubt someone deliberately planted the dangerous germ. "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.
But Stohr said the test kits are not the only supplies of the 1957 pandemic strain sitting in laboratories around the world. "The world really has to think what routine labs should be doing with these samples they have kept in the back of their fridges," he said.