Canadian scientists were the first to spot a pandemic flu strain that was mistakenly sent to labs in 18 countries, sparking the World Health Organization to issue an urgent recall Tuesday.

A U.S. company sent mislabelled samples of the H2N2 influenza that triggered a pandemic in 1957 to nearly 5,000 labs worldwide, as part of a routine testing program.

Since the strain hasn't infected humans since 1969 and it is not included in flu shots, people under the age of 37 have no immunity to H2N2.

WHO's influenza chief, Dr. Klaus Stohr.

Although the risk of a laboratory accident causing a pandemic was low, WHO urged scientists to immediately destroy the materials.

"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, Dr. Klaus Stohr, told The Associated Press.

All 20 Canadian laboratories that received the mislabelled material have destroyed it, the Public Health Agency of Canada said Tuesday.

On March 26, Canada's National Microbiology Laboratory detected the mislabelled samples, potentially averting a global problem by notifying WHO.

Laboratories worldwide use the test kits as part of their internal quality control checks, to ensure they are identifying viruses correctly.

The College of American Pathologists also uses the kits to test scientists seeking certification.

Within 24 hours, scientists worldwide are to confirm the specimens were destroyed by reporting to the College, the WHO alert said.

Lab workers who worked with the material will be monitored, and anyone showing respiratory illness will be tested for H2N2, the Canadian agency said in response to the WHO directive.

Almost 99 per cent of the labs that received the kits are in the U.S., Stohr said.

Bioterrorism is not suspected, said Dr. Nancy Cox, influenza chief at the U.S. Centers for Disease Control and Prevention in Atlanta.