Family link in bird flu deaths
If confirmed, it would be the first known case of human-to-human transmission of the virus during the outbreak sweeping Asia.
The source of the two sisters' infection has not yet been conclusively identified, said Bob Dietz, a WHO spokesman in Hanoi.
"However, WHO considers that limited human-to-human transmission from the brother to his sisters is one possible explanation," he said.
Laboratory tests in Hong Kong verified that the sisters, aged 30 and 23, had been infected by the H5N1 bird flu virus, he said.
The sisters were among eight Vietnamese whose deaths were confirmed to be from the bird flu that has infected poultry, mostly chickens, in at least 10 countries. Thailand has confirmed two human deaths from the disease.
China closed poultry markets and processing factories in bird flu-affected areas, shortly after WHO warned that Beijing's chances to contain the disease may be dwindling.
WHO on Saturday called on China to share more information about the disease, step up monitoring for possible human cases and take precautions so that workers engaged in the mass slaughter of birds are not accidentally infected.
The UN Food and Agriculture Organisation appealed for international aid for farmers across Asia, saying the farmers may otherwise resist slaughtering their flocks, a crucial measure in stamping out the disease and preventing a human outbreak. "We are concerned that mass culling is not taking place at a speed we consider absolutely necessary to contain the virus," said Hans Wagner, an FAO animal production and health officer.
Most cases have been linked to contact with sick birds, and until now no evidence of human-to-human transmission has been found.
Limited human-to-human transmission of the virus is not considered a serious danger. What experts fear is that the virus might mutate into a form that passes easily between people.
There is no evidence that a new strain has emerged, WHO spokeswoman Maria Cheng said.
Such evidence would come from tests comparing the genetic makeup of the virus found in the two sisters with that seen in other people. "This may be an isolated incident. These were very close contacts, family members," Ms Cheng said.
Vietnam now has a total of 10 confirmed cases of bird flu including the eight deaths.
An eight-year-old girl remains hospitalised in Ho Chi Minh City, while a four-year-old boy has recovered.
The two sisters from northern Thai Binh province became sick after attending their brother's wedding reception.
Their 31-year-old brother died on January 14, but was cremated so no samples were available to determine whether he also had bird flu.
Vietnam has vowed tougher measures to control the epidemic, including a nationwide ban on transporting poultry.
