The World Health Organisation (WHO) is investigating the first possible instance of "bird flu" being transmitted from person to person.
The WHO has feared that the avian flu virus could integrate with human flu to turn into a deadly and highly infectious strain of influenza that could sweep through the human population, causing a global pandemic.
The WHO's officials confirmed yesterday that two sisters in Vietnam had died of avian flu on 23 January, 11 days after their brother had almost certainly died of the same virus. His wife was also infected but has since recovered.
A detailed investigation to find out how the two sisters and their family members caught the respiratory disease has failed to find an obvious link with infected poultry, a WHO spokesman said.
"With the exception of this family cluster, all human cases in the Vietnam outbreak have been linked to contact with infected poultry," he said.
"The WHO considers that limited human-to-human trans-mission from the brother to his sisters is one possible explanation," he added.
The two women, from the northern province of Thai Binh, became sick after attending their brother's wedding reception. Their 31-year-old brother died shortly afterward but was cremated so no samples were available to determine whether he definitely also had bird flu.
Laboratory tests in Hong Kong verified that the sisters, aged 23 and 30, had been infected by the H5N1 bird flu virus, which has now infected 10 people in Vietnam, killing eight, and three people in Thailand, killing two.
Most cases in humans have been linked to contact with sick birds and, until now, there has been no evidence of human-to-human transmission.
Limited spreading of the virus between people is not considered a serious danger. What experts fear most is that the virus will mutate.
There is no evidence that a new strain has yet emerged. Such evidence would come from tests comparing the genetic make-up of the virus found in the two sisters with that seen in other people.
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