BEIJING - How H7N9 avian influenza spread to humans is hard to know as it is a newly emerged infectious virus, said a researcher of the Chinese Center for Disease Control and Prevention (CDC) on Thursday.

"The spread of H7N9 has not been completely stopped and the virus still exists in some poultry," Feng Zijian, an assistant to the CDC director, said during an online interview hosted by the National Health and Family Planning Commission.

The monitoring of H7N9 human infection should be stepped up in autumn, when flu can spread easily, so that effective measures can be taken immediately after infection was spotted, Feng said.

Poultry trade in infected areas should be restricted, and birds found infected with the virus should be culled, he said, adding that reducing human contact with poultry is a main way to control the spread of H7N9.

In areas where H7N9 previously spread, measures such as closing down poultry markets have been adopted, which effectively contained the avian flu, he added.

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 infection have been confirmed, according to the CDC.