The strain of bird flu that has killeed at least 12 in Asia cannot spread easily between humans despite one suspected case in Vietnam, the World Health Organization (WHO) said today.
But there is still a danger that the virus could mate with human strains to form a much more contagious threat, the United Nations body said.
"We do not think the current virus will mutate or change its properties to cause a very big outbreak," Mr Bjorn Melgaard, the WHO representative in Thailand, told a news conference.
"It's not a very efficient virus in terms of infecting humans. The risk is the creation of a new virus through the combination of the current avian virus with a human virus."
The WHO said on Sunday two Vietnamese sisters may have caught bird flu while looking after their sick brother, who is belived to have fallen ill after coming into contact with chicken during preparations for his wedding party.
Mr Melgaard said this suspected first case of human-to-human transmission did not mean the virus had already mutated and only a spate of human deaths would arouse fears of a new virus.
Another UN body, the Food and Agriculture Organisation, said bird flu was still on the rise, despite the slaughter of 45 million chickens in Asia.
Thai Prime Minister Thaksin Shinawatra has strongly criticised the WHO for suggesting the flu could spread to pigs. "Ethically speaking, researchers should only discuss low possibilities of such cross-strain spreads in labs, not in public," Mr Thaksin said.