BBC NEWS
Deadly flu samples still missing
Samples of a potentially lethal flu strain sent to Lebanon and Mexico did not reach the respective laboratories, the World Health Organization says.
The WHO said it was trying to trace the samples, which were sent by a US testing organisation.
The samples are of Asian flu, which killed between one and four million people in 1957 but disappeared by 1968.
More than 3,700 laboratories in 18 countries received the testing kits and have been racing to destroy the virus.
The WHO says the virus could "easily cause an influenza epidemic" if not handled properly.
All but five of the countries outside US that received the kits say they have now destroyed them.
WHERE THE VIRUS WENT
Europe: Belgium, France, Germany, Italy
Americas: Bermuda, Brazil, Canada, Chile, Mexico, the US
Asia: Hong Kong, Japan, Singapore, South Korea, Taiwan
Middle East: Israel, Lebanon, Saudi Arabia
The WHO has not said how much had been destroyed in US labs, which received the vast majority of the samples.
The College of American Pathologists (Cap) said the kits had been sent to the following countries including the US: Bermuda, Belgium, Brazil, Canada, Chile, France, Germany, Hong Kong, Israel, Italy, Japan, Lebanon, Mexico, Saudi Arabia, Singapore, South Korea, Taiwan.
But the man who co-ordinates the WHO global influenza programme, Klaus Stohr, told the BBC News website that the one laboratory in Lebanon that was supposed to have been sent the kits had not received any. And one out of four laboratories in Mexico had not had any consignment either.
Mr Stohr said the WHO and Cap were trying to find out what happened to the samples sent by prestigious international carriers.
He said it was possible that the laboratories had not gone to collect the kits. However, he said the WHO was not concerned at this stage.
"There are simpler ways of interfering with the samples" if one so wished, Mr Stohr said.
No immunity
Because the virus has not been in circulation since 1968, people born after that do not have antibodies against it - and current vaccines do not guard against it.
The Cap sent out 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.
Given the concerns that the virus could be used in bio-terrorism, letters were sent to the laboratories before the mistake was made public.
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 Centers for Disease Control and Prevention, says it was in the process of deciding whether to change the strain's classification when it found out 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.