Both lived in the northern province of Thai Binh, where a cluster of cases is causing great concern about the possibility of human-to-human transmission of the H5N1 virus, which experts fear could mutate into a form which could cause a pandemic.

"We are aware of these cases and we are investigating this further," a World Health Organization spokeswoman said.

For one of the symptom-free carriers of the virus, a 61-year-old woman, the only link established to the disease was her husband, who died of bird flu on Feb. 24.

"The old lady is totally healthy," said an official at the health clinic of her village in Quyet Tien commune.

"She took care of her husband when he was sick. Other than that, she said she ate only pork and all four chickens raised in her house tested negative for bird flu."

The other, the 81-year-old grandfather of a brother and sister who contracted the disease, drank raw duck blood during the Tet Lunar New Year festivities last month, health officials in his village said.

"It is really strange that he carries the virus for a month and did not get sick," said one health official in the Thuy Luong commune of Thai Thuy district of a virus which usually takes three to 10 days to show symptoms.

PROBE UNDER WAY

However, there was no immediate confirmation from experts that the old man had indeed been carrying the H5N1 virus for nearly a month without showing symptoms, although his 21-year-old grandson caught it after drinking raw duck blood at Tet.

The young man's 14-year-old sister was also infected after coming into contact with sick chickens and health officials confirmed this week that a nurse who tended the grandson had caught bird flu, which has killed 47 people in Asia.

Top medical officials said there was evidence the 26-year-old male nurse lived in an area where the H5N1 virus had struck poultry and that he had eaten chicken. But they said the focus of their investigation into the case was the possibility he had caught it directly from his patient.

What worries the experts most is the possibility of the virus mutating into a form which could pass easily between humans and set off a pandemic in a world without immunity to it, and kill millions.

So far, there has been no evidence that mutation has occurred and the only previous probable case of human-to-human transmission occurred in Thailand, where a mother died after cradling her sick daughter in her arms for hours.

The male nurse, being treated in a Hanoi hospital with a high fever, a symptom of the H5N1 strain which has killed more than 70 percent of the people known to have caught it, had provided bed care to the grandson.

The H5N1 virus has killed 34 Vietnamese, 12 Thais and a Cambodian since it swept across large parts of Asia at the end of 2003.

It has recurred several times despite the slaughter of millions of poultry and has spread across about half of Vietnam since the latest outbreak began in the Mekong Delta of Southern Vietnam.

It sprang up again around the same time in Thailand, where the government said it had spread to 19 of the country's 76 provinces but had not infected any more people.

Source: Reuters - 9th March 2005