By Pallab Ghosh
BBC News science correspondent
The WHO is to announce new research showing that the pattern of avian flu in northern Vietnam is consistent with human-to-human infection.
A new virus could spread quickly
Its study urges governments to bolster public health measures.
The new methods will be needed to protect against a new influenza pandemic, the WHO paper says.
It is thought that at least 92 people have caught the avian influenza virus from handling poultry since late 2003.
But in a handful of cases, there is the suspicion that the virus has mutated and spread from person to person.
Scientists fear this new infection could form the basis of a new world-wide flu pandemic.
In the first detailed assessment of this possibility, a WHO team says that the infection pattern in northern Vietnam may indicate that the infection is passing from one person to another.
1918 Spanish flu killed 50m
1957 Asian flu killed about 1m
1968 Hong Kong flu killed 1m
2003 Sars killed 774
2004-5 Avian flu (H5N1) has killed 50 to date
When the infection spreads from poultry, it usually infects a small number of shoppers or meat handlers and is quickly eradicated.
Instead, in northern Vietnam, researchers say they have discovered a higher number of infection clusters, the period of infection is longer and the age range of those infected is much wider.
The scientists have also found that the virus in northern Vietnam is genetically more different to a bird virus than other strains.
However, the WHO stresses that the pattern of infection could also be explained by a more infectious form of bird to human infection.