BBC News has apparently obtained an advance copy of a World Health Organization study which argues that human to human transmission probably occurred in Vietnam:
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.
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.