Two elderly Vietnamese who tested positive for the bird flu virus yet remained healthy pose no significant risk to other people, a World Health Organization expert said on Thursday.

“In most infections, or many infections, it is not unusual to get people who have either mild or asymptomatic infections,” Hanoi-based WHO medical epidemiologist Peter Horby told AMN Health.

“There is no evidence that asymptomatic infection like this poses any significant risk of onward transmission, so it is not alarming in that sense,” he said.

The two elderly Vietnamese, both from 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, had relatives who caught the virus.

“We certainly need to look into circumstances of how they might have got their infection, whether from poultry, a contaminated environment or from another infected person,” Horby said.

What worries the experts most is the possibility of the virus mutating into a form that 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 Thai Binh cases have prompted Vietnamese scientists to conduct tests on a wider group of contacts of the patients, at least 10 samples from relatives and people who came into contacts with the infected were being tested for bird flu, Horby said.

Results of the tests were not immediately available.

There were cases of people who caught the H5N1 virus during its first appearance, in Hong Kong in 1997, without showing symptoms but the elderly man and woman were the first known cases in Vietnam, he said.

Vietnamese health officials said the two must have had the virus, which usually displays symptoms in three to 10 days, for about a month, but Horby cast doubt on that.

“You would not expect somebody to carry influenza infection for that length of time and it is impossible to tell how long they’ve had it,” Horby said.