And an official of the World Health Organization's European operation said the body is closely monitoring the investigation into the events that took place at Baxter International's research facility in Orth-Donau, Austria.

"At this juncture we are confident in saying that public health and occupational risk is minimal at present," medical officer Roberta Andraghetti said from Copenhagen, Denmark.

"But what remains unanswered are the circumstances surrounding the incident in the Baxter facility in Orth-Donau."

The contaminated product, a mix of H3N2 seasonal flu viruses and unlabelled H5N1 viruses, was supplied to an Austrian research company. The Austrian firm, Avir Green Hills Biotechnology, then sent portions of it to sub-contractors in the Czech Republic, Slovenia and Germany.

The contamination incident, which is being investigated by the four European countries, came to light when the subcontractor in the Czech Republic inoculated ferrets with the product and they died. Ferrets shouldn't die from exposure to human H3N2 flu viruses.

Public health authorities concerned about what has been described as a "serious error" on Baxter's part have assumed the death of the ferrets meant the H5N1 virus in the product was live. But the company, Baxter International Inc., has been parsimonious about the amount of information it has released about the event.

On Friday, the company's director of global bioscience communications confirmed what scientists have suspected.

"It was live," Christopher Bona said in an email.

The contaminated product, which Baxter calls "experimental virus material," was made at the Orth-Donau research facility. Baxter makes its flu vaccine - including a human H5N1 vaccine for which a licence is expected shortly - at a facility in the Czech Republic.

People familiar with biosecurity rules are dismayed by evidence that human H3N2 and avian H5N1 viruses somehow co-mingled in the Orth-Donau facility. That is a dangerous practice that should not be allowed to happen, a number of experts insisted.

Accidental release of a mixture of live H5N1 and H3N2 viruses could have resulted in dire consequences.

While H5N1 doesn't easily infect people, H3N2 viruses do. If someone exposed to a mixture of the two had been simultaneously infected with both strains, he or she could have served as an incubator for a hybrid virus able to transmit easily to and among people.

That mixing process, called reassortment, is one of two ways pandemic viruses are created.

There is no suggestion that happened because of this accident, however.

"We have no evidence of any reassortment, that any reassortment may have occurred," said Andraghetti.

"And we have no evidence of any increased transmissibility of the viruses that were involved in the experiment with the ferrets in the Czech Republic."

Baxter hasn't shed much light - at least not publicly - on how the accident happened. Earlier this week Bona called the mistake the result of a combination of "just the process itself, (and) technical and human error in this procedure."

He said he couldn't reveal more information because it would give away proprietary information about Baxter's production process.

Andraghetti said Friday the four investigating governments are co-operating closely with the WHO and the European Centre for Disease Control in Stockholm, Sweden.

"We are in very close contact with Austrian authorities to understand what the circumstances of the incident in their laboratory were," she said.

"And the reason for us wishing to know what has happened is to prevent similar events in the future and to share lessons that can be learned from this event with others to prevent similar events. ... This is very important."

----------------------------