So far, laboratories in 12 of the 18 countries that inadvertently received the samples containing H2N2 "Asian" flu from the College of American Pathologists (CAP) have destroyed them, the World Health Organisation said on Friday.
"Some of the countries and laboratories never received anything," said Klaus Stohr, the WHO's chief flu expert.
"They were on the address list of the college, but never received anything. We were given to understand that the material was shipped, but it never arrived, for instance in Lebanon, Mexico," he added.
Two-thirds of the labs have
incinerated the samples
Stohr said he placed a very high priority on an investigation in the United States to find out if there was an error in the shipping list, or if the samples went missing during shipment.
"There is still a possibility that this material was never sent, but there is no confirmation."
The H2N2 strain is similar to the 1957 flu virus that killed up to four million people across the world in the late 1950s, and as such should not have been sent out in routine test kits for pathogens.
Stohr played down the health risks, because the samples were freeze dried and swiftly deteriorated on exposure to room temperatures, water or sunlight.
The discovery in recent weeks of the shipments since October sparked a WHO alert and a scramble by the 3,747 laboratories, which are mainly in the United States, to destroy them.
US health authorities and the Atlanta-based Center For Disease Control were at the heart of the probe into the shipments by the College.
"There is still a possibility that this material was never sent, but there is no confirmation"
WHO chief flu expert
"It's obvious that the end of this investigation by the CDC and our association is to look at what really went wrong and why this virus strain was sent out ... those questions have to be answered," said Dr Jared Schwartz, spokesman for CAP.
The WHO and US authorities said two thirds of the laboratories (2227) had confirmed that the samples had been incinerated, including more than 90% of the 60 laboratories outside the United States.
CAP is one of four organisations in the United States authorised to regularly test laboratories' capacity to recognise a pathogen by selecting certain types of virus.