The US government has told more than 3,700 laboratories in 18 countries to destroy potentially lethal influenza samples sent out in testing kits.

The samples are of "Asian flu", which killed more than one million people in 1957 but disappeared by 1968.

Klaus Stohr of the World Health Organization (WHO) told the BBC that people born after 1968 did not have antibodies against the virus.

"If the virus gets loose, it can easily cause an influenza epidemic," he said.

"If this virus were to infect one person, it would spread very rapidly."

The College of American Pathologists sent out the kits between October 2004 and February of this year.

On 8 April, the US government asked the body to write to the laboratories affected - of which 61 are outside the US and Canada - telling them to destroy the samples.

The risk is considered to be low... but as long as this is out, it is possible laboratory technicians can become infected
Klaus Stohr, WHO

Given the concerns that the virus could be used in bio-terrorism, letters were sent to the laboratories before the mistake was made public.

Dr Stohr said the CAP had not violated US regulations, which are now being revised.

The virus - technically known as H2N2 - was classified as Biological Safety Level 2, meaning that it was not considered particularly dangerous.

But the US government agency responsible for classifying viruses, the Center for Disease Control and Prevention, says it was in the process of deciding whether to change the strain's classification when it found out that it had been widely circulated.

The WHO says there is no guarantee that every sample of the virus can be traced and destroyed because some of the laboratories may have sent derivatives of the sample elsewhere.

But there have been no reports of anyone becoming ill from handling the virus, which the WHO called reassuring.

"The risk is considered to be low... but as long as this is out it is possible laboratory technicians can become infected," Dr Stohr said.

Laboratories use the kits to show they can correctly identify different strains of a virus.

They normally include strains in current or recent circulation.

It is hoped the laboratories will have destroyed the vials by the end of the week.

The UK is not among the countries to which the strain was sent.

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Therefore, if it was likely to escape, anyone born before 1967 would have some level of immunity to it and those born after would be far more susceptible. Bit of a boobo!