The two men, ages 27 and 87, died in Shanghai earlier this month after being infected with H7N9, the first such cases of infection of humans from this rare strain, but health officials said no bird flu virus was detected in tests of dead pigs found in the Huangpu River which provides drinking water to Shanghai residents, Xinhua News Agency reported.
China's National Health and Family Planning Commission said the two men became ill with fever and coughs in late February before dying, and had suffered severe pneumonia and breathing difficulties.
They were confirmed to have been infected with H7N9 avian influenza by an expert team based on clinical observation, laboratory tests and epidemiological surveys, the report said.
A third person, a 35-year-old woman in Chuzhou City in eastern Anhui province also was confirmed to have contracted the H7N9 virus on March 9 and remained in critical condition at a hospital, Xinhua said.
The report said H7N9 bird flu virus shows no signs of being highly contagious among humans and there are no vaccines against it anywhere as not much research has been done on the virus.
Health authorities said the Shanghai Animal Disease Prevention and Control Center tested 34 pig carcasses pulled from the river but found no bird flu viruses.
The discovery of thousands of dead pigs in the river last month had, however, crated panic among residents, the report said.
The commission said it remained unclear how the three persons got infected. Its experts said there were no signs they contracted the disease from one another nor were there any signs of any infected person being in close contact with any of them.
The New York Times has reported the H7N9 strain had not been previously found in humans. However, another strain of avian flu, H5N1, has led to hundreds of deaths since 2003 and has killed millions of birds, the report said.