While the study cannot prove these people were infected with bird flu, it suggests that infections may be going undetected, said Dr. Anna Thorson of the Karolinska Institutet in Stockholm, who led the study.

"The verified human cases of highly pathogenic avian influenza in Vietnam may represent only a selection of the most severely ill patients," Thorson's team wrote in their report, published in the Archives of Internal Medicine.

H5N1 avian influenza was first reported in people in Hong Kong in 1997, when it infected 18 people and killed 6. It re-emerged in 2003 and has infected at least 156 people and killed about half of them - including 93 infections and 42 deaths in Vietnam, according to the World Health Organization.

But that is only the number of cases that have been diagnosed and confirmed. Half of all respiratory deaths are never diagnosed in developed countries and the numbers are even lower in developing nations.

While H5N1 remains primarily a disease of birds, experts fear it could acquire the ability to pass easily from person to person, sparking a pandemic that could kill millions.

Thorson's team decided to gather evidence that avian flu may have been circulating undetected in Vietnam.

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They sent trained interviewers to a rural Vietnamese region with confirmed outbreaks of H5N1 avian influenza among poultry. "We included 45,478 randomly selected inhabitants," they wrote.

They found that 8,149 people, or close to 18 percent, reported having some sort of flu-like illness and that about 25 percent of all those surveyed lived in households reporting sick or dead poultry.

"The flu-like illness attributed to direct contact with sick or dead poultry was estimated to be 650 to 750 cases," the researchers said.

"Our epidemiological data are consistent with transmission of mild, highly pathogenic avian influenza to humans and suggest that transmission could be more common than anticipated, though close contact seems required," they concluded.

"Having poultry in the household was in itself not a risk factor for flu-like illness, but contact with sick or dead poultry was," they added.

They said someone should follow up with blood tests to see if people who reported flu-like illness had antibodies to H5N1, which would indicate they had been infected with the virus at some point.

If the virus is circulating more widely among people than believed, this could be dangerous because it would give the virus more opportunities to evolve into a form that more easily infects humans, the researchers said.