The killer “Asian” flu virus, which was sent to laboratories around the world as part of routine test kits, could trigger a pandemic if it escapes, but the chances of that are low, the World Health Organization said on Wednesday.

Senior WHO scientist Dr. Klaus Stohr said the H2N2 virus, which killed between 1 million and 4 million people in 1957, had gone to about 3,700 laboratories, nearly all in the United States.

“The virus could cause a global (flu) outbreak. It was an unwise decision to send it out,” said Stohr, who heads the United Nations health agency’s influenza program.

But the laboratories, which are sent viruses to test their ability to detect strains, are experienced in handling such material and most had already been alerted to the danger, so there was little chance of anyone catching it, he added.

“It is a risk, but it is considered low. It should not lead to a big scare,” Stohr told Reuters.

The U.S. organisation that sent out the virus, the College of American Pathologists (CAP), has issued instructions for all samples to be destroyed and would report to the WHO and U.S. health authorities by Friday on the response, he said.

“By Friday we may be through with this,” Stohr said.

Test kits sent to labs so they can check their detection proficiency normally involve viruses that are in circulation among the world’s population. It was not immediately clear why the U.S. concern had opted to take a lethal strain out of cold storage.

NO IMMUNITY

The problem with the 1957 H2N2 virus was that nobody born after 1968 would have much immunity to it.

The virus, whose appearance nearly 40 years ago coincided with the so-called Asian influenza pandemic, is spread easily between humans. It continued to cause annual epidemics for a decade, when it vanished with the emergence of a new virus. As a result, it was no longer considered necessary to include it in vaccines.

But Stohr said that the laboratory scare was a one-off because there were no other such lethal flu viruses being preserved by medical institutions.

The strain that may have killed up to 40 million people in 1918 - the so-called Spanish flu - had long since disappeared and the virus of the last great influenza epidemic of 1968 was still in circulation, meaning that people had natural resistance.

PUBLIC ANNOUNCEMENT DELAYED

The United Nations health agency was first alerted on March 26 by the Canadian Public Health Agency to the fact that the H2N2 virus had been detected.

It was found because there had been cross-contamination with other flu samples in a Canadian laboratory, but the source was only identified as being the proficiency kit last Friday.

The WHO said that it had delayed going public with the news about the virus for security reasons, so that laboratories could have time to make the samples safe.

The microbes went to some 61 laboratories outside North America, all of which had been contacted, Stohr said. But it was not certain that all U.S. recipients had been located yet, Stohr said. “There is more detective work to be done there,” he added.

So far laboratories in Canada, South Korea, Hong Kong and Singapore had reported back that all the virus received had been destroyed, Stohr said.

He noted that the first batches had been delivered as long ago as last October and the fact that no infections had been reported was a good sign.

The H2N2 virus - which was considered “mild” as pandemics go - had mainly attacked people over 60 and it would have been quickly picked up by health authorities if there had been any contagion, Stohr told a news conference.

“We are a little bit fortunate this has not gone worse. But then fortune sometimes goes along with proficiency,” he added, noting that all involved had responded quickly to the threat.

Countries outside the United States that had received samples for testing included Saudi Arabia, Jamaica, Mexico, Lebanon, Brazil, Hong Kong and Italy, Stohr said.