The new H7N9 virus that has emerged on the mainland in recent months is more likely to be transmissible between humans than any other known bird flu virus, Chinese and world experts have concluded after investigations in affected areas.
They based their conclusion on the fact that the virus has caused more human infections in a shorter period than any other bird flu virus, and that it has undergone genetic changes that suggested it is better adapted to infecting humans.
"The virus appears to be very transmissible from poultry to human, implying that it could more easily become transmissible between humans," said Professor Malik Peiris, head of virology at the University of Hong Kong and one of 14 experts who took part in the study.
The findings were published last month after a joint mission by the World Health Organisation, Chinese health authorities and international influenza experts spent a week in Beijing and Shanghai in April.
There were 132 cases of H7N9 human infections reported from February to May, a much higher rate of infection than H5N1 virus, which was responsible for about 60 cases a year over the past 10 years. In the first human outbreak of H5N1 in Hong Kong in 1997, 18 cases were reported, according to the WHO.
There have so far been no cases of human-to-human transmission of either H7N9 or H5N1, but H7N9 has mutated genetically to allow it to bind to human cells, while H5N1 has not.
Another strain named H9N2 has some of these mutations but it very rarely infected humans, Peiris said.
The joint mission report recommended ongoing monitoring of human and animal populations in all mainland provinces to identify any signs that the H7N9 virus was spreading geographically, gaining the ability to infect humans more easily, or transmitting efficiently among humans.
An H7N9 vaccine is being developed by the WHO.
The virus appears to have become less active, with no new infections in the past month. Peiris said it was likely that intervention and the virus' seasonal pattern played a role in curbing it.
Mainland authorities have closed live poultry markets in Shanghai and other affected areas and there have been no new cases since.
Bird flu is less active in warmer weather, but cases go up as the temperature falls.
Peiris said: "What many of us believe is that H7N9 will not completely disappear in poultry. It may appear again in winter."
View H7N9 map in a larger map
Click on each balloon for more information on individual patients infected: blue, patients infected with the H7N9 virus under treatment; red, those infected with H7N9 who have died; yellow, those who have fully recovered; and pink, those infected other types of the Influenza A virus, including H1N1.