The World Health Organization says tests show no sign that the bird flu virus in Asia has changed to a form easily transmissible between humans. But human-to-human transmission has still not been ruled out.
U.N. scientists say a bird flu virus that killed three members of a Vietnamese family last month had not changed into a strain that could easily be passed between people.
The scientists originally feared that two sisters in the family who died of the bird flu might have caught the virus from their brother, who also died of the disease.
That could have signaled that the avian disease had picked up genes from a human influenza virus. If that happens, a new human-to-human strain of the virus could emerge, possibly of the type that killed millions of people during pandemics in the last century. But the scientists said their latest tests showed the virus carried no human flu traits.
But Maria Cheng, a WHO spokeswoman based in Hanoi, says there is still the possibility that one member of the Vietnamese family passed the avian strain of the virus to another.
"This still does not rule out the possibility of human-to-human transmission, but the important thing is it shows there has not been the emergence [yet] of a new strain," she said.
She warns that such an emergence is still a possibility, and the danger of a pandemic remains.
The dangers of this particular type of bird flu virus, known as H5N1, were first highlighted in 1997 when it crossed into humans in Hong Kong. Eighteen people contracted the disease then, and six of those died.
The bird flu epidemic that has been devastating Asia's poultry flocks in recent weeks has led to at least 19 human deaths in Vietnam and Thailand. Indications are that all those victims contracted the disease from infected birds.
Governments in at least 10 Asian countries are attempting to contain the spread of the disease, and tens of millions of chickens have already been slaughtered.
WHO spokeswoman Cheng says it could take as long as two years to eradicate the disease from poultry in the region.