The WHO issued the alert after U.S. health officials said samples of the H2N2 influenza virus were recently sent out to more than 4,000 labs, most of them in the United States.
The H2N2 virus killed between 1 million and 4 million people worldwide during the Asian influenza pandemic of 1957-58. The virus disappeared in 1968, and has not been included in flu vaccines since then.
Klaus Stohr, the WHO's top influenza expert, warned yesterday that people born after 1968 would probably have no immunity -- or very limited immunity -- to the strain.
"This virus since  has not been circulating in humans," Stohr said. "This virus is fully transmissible from human to human, and everybody born after 1968 would have no immunity."
The H2N2 samples began to be sent out last September, as part of a testing process that measures a laboratory's proficiency in detecting various strains of influenza.
CAP issued an alert last week after a laboratory in Canada detected H2N2.
It is unclear, however, how the strain ended up in the test panels.
The College of American Pathologists (CAP) directs the testing and has a policy of excluding from test samples viruses that are potentially fatal to humans.
Dr. Jared Schwartz, a CAP representative, said the U.S. company responsible for sending out the samples thought it had sent out an ordinary flu strain and not H2N2.
Schwartz said the company, Meridian, found the virus in 2000 in a so-called "germ library" that had come from another firm.
A Meridian spokeswoman said company executives were not available for comment.
Scientists rank viruses based on the level of safety precautions that must be taken when handling them. Extremely dangerous viruses, like Ebola, can only be handled at labs with top-level safety measures. Those labs have a bio-safety level, or BSL, of 4.
The H2N2 has long been a level-2 virus, but many countries have upgraded it to a level of 3 because so many people have no immunity to it.
The WHO’s Klaus Stohr said the decision to include the H2N2 virus in the kits was not “a good idea.”
"The company (CAP) decided to include this BSL2 virus H2N2 in the shipment," Stohr said. "Legally, that's fine. Epidemiologically, and looking at the risk assessment, it may have not been a good idea to do that. It is certainly something which will have to be reconsidered in the future, definitely. WHO will make recommendations on the use of any H2N2 virus in the future."
The WHO, however, downplayed the risk of anyone being infected and reported that no infections had been detected. WHO spokesman Ian Simpson told RFE/RL the risk posed by the virus is small.
"It's possible that this virus, if it escapes from the laboratories where it's been sent, could cause a serious flu outbreak in the world," Simpson said. "But at this point, there is no evidence that the virus has escaped from the laboratory, and the laboratories where it's been sent are all laboratories which are used to dealing with influenza viruses. And so, if normal procedures are followed, there should be minimal risk."
Still, WHO officials instructed labs to destroy the samples as quickly as possible in order to avoid any risk.
Health authorities in Brazil, Canada, Chile, Hong Kong, Mexico, Singapore, South Korea, and Taiwan have already reported that labs in their countries had destroyed the samples. Japan said it ordered nine labs to destroy them.
The U.S. Centers for Disease Control and Prevention (CDC), said it contacted all the U.S. labs that had received the samples.
Simpson said all kits should be destroyed within a week.
"The laboratories have been asked to destroy these stocks within seven days and we're working with the national authorities in the countries where the laboratories were sent the sample, to insure that all the samples are identified and destroyed as rapidly as possible," Simpson said.
The emergence of the H2N2 germ in labs throughout the world has again raised questions about how to safely handle lethal viruses. It's an issue that led to tougher U.S. regulations after anthrax was transmitted via postal mail in 2001, killing five Americans.
White House spokesman Scott McClellan, while playing down the H2N2 risk to the public, said U.S. President George W. Bush considered the destruction of the virus samples a "high priority."
Could H2N2 be used as a form of bioterrorism? Georg Pauli, a scientist at the Robert Koch Institute of Public Health in Berlin, told RFE/RL the chances of a flu virus being used by terrorists are very limited.
"A lot of people are also thinking about influenza as a bio-terrorists’ weapon," Simpson said. "But up to now, the surveillance of this virus is very strict all over the world, so one would detect a new strain very rapidly. That is different from the other agents, like for instance, anthrax."
Furthermore, Pauli said, since H2N2 was used in vaccines before 1968, a vaccine could be produced again quite quickly.