The World Health Organization said Sunday that two sisters who died in Vietnam's Thai Binh province last week were victims of avian influenza and might have contracted the disease from their brother.
If confirmed, the sisters' deaths would represent the first human-to-human transmission of the illness during this outbreak. Until now, health officials had concluded that human victims in Vietnam and Thailand were contracting the virus through direct contact with infected fowl.
The deaths of the two women, ages 23 and 30, bring to 10 the number of confirmed bird flu deaths in Vietnam. There also have been two confirmed deaths in Thailand from the virus, which has hit 10 Asian nations and killed millions of chickens and ducks.
"The investigation has not been able to conclusively identify the source of infection for the two sisters," the World Health Organization said in a statement issued by its Hanoi office. "However, WHO considers that limited human-to-human transmission, from the brother to his sisters, is one possible explanation."
While the number of human victims of avian flu has been relatively small, health experts fear that if the virus gains a foothold in the human population it could develop the ability to spread easily among people, potentially killing millions worldwide.
During a 1997 bird flu outbreak in Hong Kong that killed six, doctors found "similar instances of limited transmission" of the virus between humans, but those cases never developed into a significant health threat, WHO said.