All laboratories that received a killer flu bug in test kits have destroyed the virus, except for a few in the United States where the job should be done in a day or two, the World Health Organization (WHO) said Monday.

“We think that it should all be done within the next couple of days,” Maria Cheng, a WHO spokeswoman, told Reuters.

Over 3,700 centers in 18 countries were sent samples of the H2N2 virus, which killed between one and four million people in 1957, as part of routine testing of laboratories’ ability to detect strains.

The decision by the U.S. College of American Pathologists (CAP) to use the “Asian” flu virus was criticized as “unwise” by the WHO, which feared it could trigger a pandemic if it escaped from the laboratories.

Cheng said there were no reports of anybody being infected by the virus, against which few people would have immunity because it has not been in circulation since 1968.

First samples were sent out last October. The order to destroy them went out over a week ago after a laboratory in Canada sounded the alarm.

Samples initially listed as having been sent to laboratories in Lebanon, Mexico and Chile, but which apparently never arrived, causing concern that they might have gone astray, had also been tracked down and destroyed, Cheng said.