Two Vietnamese sisters, who died of bird flu, may have caught the virus from their brother, who had also died, the World Health Organization said in a statement on Sunday.

While the source of infection for the two sisters cannot be conclusively identified, WHO "considers that limited human-to-human transmission, from their brother to the sisters, is one possible explanation," said WHO.

The statement said that the two women, aged 23 and 30 years old, were admitted to the Hanoi-based Hospital of Tropical Diseases on Jan 13. Both died on Jan. 23.

They are part of a cluster of four cases of respiratory illness: the two sisters, their elder brother and his wife in the northern province of Thai Binh. Their brother died before them, but no samples were available from him for testing.

A detailed investigation of this cluster has been undertaken by WHO.

Vietnam's Hygiene and Epidemiology Institute had confirmed thatthe two sisters were victims of the highly pathogenic H5N1 form of the virus before WHO did.

The WHO investigation did not reveal a specific event, such as contact with sick fowls, or an environmental source to explain the cases. However, WHO stated that H5N1 infection in poultry is widespread in Vietnam, and that direct transmission from fowls to humans cannot be ruled out on the basis of available evidence.

"At present, there is no evidence of efficient human-to-human transmission of H5N1 occurring in Vietnam or elsewhere. Further investigations will be carried out, but it is doubtful that they will provide any more conclusive information about how these people became infected," said WHO.

Vietnam's Central Veterinarian Department said that by Sunday afternoon, 47 out of 64 cities and provinces nationwide were hit by bird flu which had affected nearly seven million poultry.

(Xinhua News Agency February 1, 2004)