BEIJING - Chinese scientists have discovered that a type of H7N9 can bind with a human receptor, which explains how the virus can infect human beings.

Research focused on how H7N9 infected human beings, examining the two earliest reported virus types, known as isolates. These were SH-H7N9 and AH-H7N9, discovered in Shanghai and Anhui province respectively.

Researchers evaluated the viral hemagglutinin receptor binding properties of the two isolates. A receptor is a protein molecule in a cell, or on a cell, to which a substance can bind. A virus has to combine with a receptor in order to infect the host.

Researchers found that SH-H7N9 (reported in Shanghai) preferentially binds the avian receptor analog, whereas the AH-H7N9 (reported in Anhui) binds both avian and human receptor analogs.

The research titled "Structures and receptor binding of hemagglutinins from human-infecting H7N9 influenza viruses", jointly authored by a team from the Chinese Academy of Sciences, was published online on September 5 by Science, a leading journal.

Shi Yi, researcher with the academy and lead author of the paper, said AH-H7N9 was prevalent during the outbreak, whereas SH-H7N9 was only isolated from one case.

Researchers warned the H7N9 virus must be closely monitored in case of mutation.

H7N9 bird flu has killed 45 people on the Chinese mainland since the first human infection was confirmed in late March. A total of 134 cases of H7N9 human infection have been confirmed, according to China's health authorities.