"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.