A researcher with the World Health Organization says people who test positive for bird flu but have no symptoms of the disease may be contagious, but probably pose a lower risk of transmission to others.

Peter Horby, an epidemiologist based in Hanoi, made the comments a day after the grandfather of two bird flu patients tested positive for the virus, despite never falling ill.

It is not clear whether he caught the disease from family members or infected poultry.

Experts are concerned about a worldwide pandemic if bird flu changes to a form easily transmitted between people.

Some 46 people have died of the H5N1 bird flu virus since it reemerged in large parts of Asia in late 2003. An earlier outbreak began in Hong Kong in 1997.