The deadly H7N9 influenza spreads at unprecedented speeds, Hong Kong microbiologists say, faster than all known strains of the bird flu virus.
The bug - which infected 130 people on the mainland in March and April, killing more than 40 - may derive its speed from being more readily transmitted from birds to humans than other strains, including H5N1.
A study by University of Hong Kong Professor Yuen Kwok-yung and his team draws a comprehensive comparison between the two viruses, including their structure, mode of transmission and at-risk groups.
It emphasises that the new virus has proved more deadly and harder to control than H5N1, despite resembling it in many ways.
Stringent measures to control the poultry outbreak - such as culling birds and closing wet markets - should be put in place on the mainland to prevent a pandemic, the scientists warn.
The team says in an article in the latest The Lancet medical journal that the pace of H7N9's spread could be related to speedier transmission from poultry to humans or improved testing that detects it more quickly.
The two viruses have genetic markers showing adaptation to mammals and have improved both their binding to human-type receptors and their replication in mammals.
But H7N9 seems to be more readily transmitted from poultry to people.
They say there is "no definitive evidence" yet to suggest continuing person-to-person transmission of H7N9.
They say the development of poultry and human H7N9 vaccines is important as they expect increasing problems with bird flu in poultry and humans.