The World Health Organization says thousands of laboratories around the world have been told to destroy samples of a deadly flu strain that was inadvertently sent to them as part of a routine testing procedure. The agency says the virus could set off a pandemic, if it escaped the laboratories.

The director of WHO's Global Influenza Program, Klaus Stohr, says the H2N2 virus is of concern, because it is very similar to a virus, which caused a global influenza outbreak in 1957. He says the virus stayed around until 1968, when it disappeared. He says this virus has not been circulating among humans since then.

"This virus is fully transmissible from humans to humans, and everybody born after 1968 would have no immunity. This H2N2 virus was, by this company in the United States, distributed to more than 3,700 laboratories worldwide. The majority of thesesamples were sent to American laboratories, in Canada, 14 laboratories, and 61 laboratories outside North America in 16 countries," he said.

The College of American Pathologists sent the test kits containing the sample of the deadly flu strain to the laboratories in February. This is done routinely about three times a year, so labs can check their quality control. The Public Health Agency of Canada informed the World Health Organization on March 26 that a local laboratory had discovered and identified the deadly strain in the kit it had received.

Dr. Jared Schwartz, a spokesman for the College of American Pathologists, said the virus was distributed in error.

"It's unfortunate, regrettable. We wish it never happened," he said. "We never would have asked for a H2N2 virus to be sent to our customers."

Dr. Schwartz and Dr. Stohr both said the risk that a laboratory worker would get sick from the virus and transmit the illness to others is very low. But, were this to happen, the consequences could be very serious. Dr. Stohr says the 1957 influenza pandemic killed between one million and four million people.

Dr. Stohr says WHO took urgent action when it learned of the virus, and the College of American Pathologists instructed all the laboratories, which had received the test kits, to destroy the virus. Dr. Stohr says WHO delayed making the information public, until it was sure that work on the destruction of the virus had actually started.

"Because this H2N2 virus could possibly also be used for other purposes. There is a bio-security risk. We did not want to arouse interest in, perhaps, in a way that could lead, provide access to others to this virus, … whom we may not want to have access to it. With the undertaking being in full swing, we felt yesterday it is a good time to go out [with the information]," he added.

Since the September 11, 2001, terrorist attacks in the United States, fear of bioterrorism has increased. WHO does not see anything sinister in the discovery of the deadly virus. But, Dr. Stohr agrees sending the sample was, as he says, not such a good idea.

He says Canada has destroyed all the samples it received, as have South Korea, Hong Kong, Singapore and many labs in the United States. He says Taiwan also is moving fast to get rid of the virus. He says WHO should have a fuller picture of the situation by Friday.

In Washington, a White House spokesman said the Centers for Disease Control is working with the labs, and although the risk is low, the U.S. government does not want to "take any chances."