A World Health Organisation study in Vietnam has raised the possibility that bird flu is becoming more capable of human-to-human transmission, the WHO said.

The study of bird flu outbreaks in Vietnam until April 2005, presented to a meeting with Asian countries in Manila on May 6, suggested an evolution of infections by the H5N1 virus in Vietnam, according to a report on the meeting.

"The changes in the epidemiological patterns are consistent with the possibility that recently emerging H5N1 viruses may be more infectious for humans," it said.

While that meant a greater number of people might be infected by poultry, there was also evidence that human to human transmission, which has already been found several times since the strain was first detected in Hong Kong in 1997, was strengthening, the UN health agency said.

"It is possible that avian flu viruses are becoming more capable of human-to-human transmission," the report said.

"While the implications of these epidemiological and virological findings are not fully clear, they demonstrate that the viruses are continuing to evolve and pose a continuing and potentially growing pandemic threat," the report said.

Fears of that a new deadlier strain of flu might spread rapidly around the world on a similar scale to pandemics in the last century have been revived with the emergence of the H5N1 bird flu virus among humans in Asia.

Fifty-two people have died in Vietnam, Thailand and Cambodia since late 2003, mainly through infection from poultry.

Concern is now heightened because a pandemic strain may develop through a series of small steps that taken individually might not be enough to signal clearly that an epidemic was about to start, according to the UN health agency.

"It is not sudden event but a complex sequence of events that lead to a pandemic," said WHO official Richard Nesbit.

The pattern of changes observed in north Vietnam included more and larger human clusters of the disease, the increasing mean age of the victims and the lower fatality rate.

Analysis of genes from both avian and human forms of H5N1 from several countries also suggested changes in the virus, the report said.

The report reiterated the health agency´s call for "immediate steps" to boost monitoring for possible pandemic influenza in all countries affected by H5N1 in birds.

It also recommended that all countries, including those which were unaffected by the strain, should move ahead with operational plans to tackle a possible global spread of deadlier form of flu.

Other interpretations for the trends observed in Vietnam were raised, including transmission through contaminated water or food or infection from poultry that carried the virus but did not show symptoms, or greater persistence of the virus in the environment.

The experts meting in Manila also considered that the changes seen in north Vietnam could be "inconsequential" but dismissed that assessment as unlikely.

A second human case of bird flu was identified in under a week in Vietnam on Tuesday.