A joint study by the World Health Organisation and the central government said poultry farms might be the source of the H7N9 virus that has killed 21 people.
The study, compiled after a week-long field assessment of the influenza by a panel of experts commissioned by the WHO, was publicised by the National Health and Family Planning Commission yesterday.
The report said H7N9 had a higher potential for human-to-human transmission than any other known bird-flu virus, but added that there was no evidence supporting that such transmission had already taken place.
Apart from the three family clusters reported by the commission earlier, all the remaining cases were sporadic.
It said H7N9 had infected more patients in a shorter time than other bird-flu viruses, and some samples had shown genetic alterations, which meant the organism had adapted to be more contagious than other avian-influenza viruses.
Among the 104 confirmed cases, 72 per cent have had exposures to poultry and poultry markets. It said poultry farms were likely the source of infections.
"Although the virus was not found in poultry farms yet, they are likely the source of the virus, which spread further in the live-poultry markets and eventually infected humans."
The report said it was uncertain why there were more elderly male patients than others, and the researchers were not sure if this was related to the behavioural patterns of older men.
The WHO offered the central government several suggestions, including staying alert despite the virus' seasonal weakening during the summer, as the disease posed grave hazards and much basic information was still not known, such as how long the organism has existed and whether some human cases were undetected because the symptoms were mild.
The WHO last month sent a joint mission of experts to China to survey areas affected by H7N9 in Shanghai and Beijing for a week-long assessment.
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.