A string of bird flu (search) infections in northern Vietnam (search) involving several families has raised troubling questions over whether the deadly virus that has killed 46 people in Asia may be changing, health experts said Thursday.

Six of the last seven cases confirmed in the past couple weeks are from northern Thai Binh province (search) and are connected to two families — with some relatives showing no symptoms of the disease.

Health experts are unsure whether the new cases signal the possibility that the virus is slowly transforming to allow human-to-human transmission or just underscore more careful detection.

There has been no evidence so far that the virus has mutated into a form easily transmitted by people. Health officials warn that when that happens, the virus could spark a global pandemic that kills millions.

"Certainly, it calls for investigation. It's too early to say whether these cases are any different from previous cases or not," said Dr. Peter Horby, a Hanoi-based epidemiologist for the World Health Organization.

"The two people with atypical infections could be related to improved testing, or it could relate to some difference in the virus. We won't know till we isolate the virus. It's too early to be raising alarms," he said, referring to the two latest cases.

In those cases, an 80-year-old man, whose two grandchildren are hospitalized with the bird flu, and a 61-year-old woman, whose husband died from the disease, both tested positive for the H5N1 virus but showed no signs of illness.

The mild or even symptomless infections raise the possibility that the H5N1 virus is changing, and that there may be more undetected cases.

"Undoubtedly, there are more cases than those we detect... I'm sure there's cases we're missing," Horby said.

Vietnam has had a total of 33 deaths from the deadly disease, 13 of them in the latest outbreak which began last December. A Cambodian woman also died this year in a Vietnamese hospital, while 12 people have died in Thailand.

Several cases in Vietnam and Thailand have involved families, though only a Thai case involving a mother and daughter has been proven to be caused by limited human-to-human infection.

Vietnam has detected five separate family clusters since 2003, four of them in Thai Binh. In addition to the two recent incidents, there was a case involving three brothers who ate duck blood pudding, three siblings who died after slaughtering chicken for a wedding, and a mother and daughter from southern Vietnam who died.

Vietnamese officials have said they suspect all those infected contracted the virus through contact with sick poultry, but they cannot discount the possibility that the family members may have infected each other.

"We have not been able to confirm on how these people got infected. No one has ruled out the possibility of human-to-human transmission," said Pham Van Diu, director of Preventive Medicine Center in Thai Binh.

Earlier this week, the WHO reported that seven cases which had previously tested negative in January were later found to be positive after being retested in Tokyo. Those patients, all from southern Vietnam, later recovered. Laboratory tests showed the virus had not changed its form.

The WHO said it would wait for additional details, as well as case outcomes, before adding those seven to the official count.

However, Thursday's Youth newspaper said the Pasteur Institute in Ho Chi Minh City, which originally conducted the testing, and Vietnam's Ministry of Health did not recognize the new results by the WHO lab.

The institute said the same samples sent to a different laboratory outside the country also had a negative reading.