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There is no evidence so far of sustained human-to-human transmission of H7N9 bird flu virus, World Health Organization representative to China Michael O'Leary said at a news briefing on Sunday in Beijing.

O'Leary's comment followed reports of a married couple in Shanghai being infected with the virus.

The 56-year-old man was diagnosed with H7N9 bird flu after his wife died from the infection.

It's difficult to determine whether the man got the virus from his wife or from the same source of infection his wife was exposed to, "but that's part of the very active investigation", said O'Leary.

If there are only very rare cases of human-to-human transmission, that is different from sustained person-to-person transmission, he said.

Sustained human-to-human transmission raises the risk of a pandemic, medical experts said.

"It's that easy and sustained transmission among humans that we are concerned about, and there is no evidence of that yet," O'Leary said.

With influenza, for example, 20 to 30 percent of a patient's family members could be expected to develop the illness, according to medical experts.

Authorities have been investigating cases of human infection with H7N9 within a family or involving people unexposed to birds.

Other key information like the transmission dynamics, modes of transmission, the scope of the problem and the severity also needs to be clarified through investigation, said Feng Zijian, director of the emergency response department at the Chinese Center for Disease Control and Prevention.

Feng said that sporadic individual cases of human-to-human transmission would not necessarily mean the virus can be transmitted effectively among humans.

Surveillance of people who have close contact with patients, particularly family members and healthcare providers, will be enhanced to help determine whether human-to-human H7N9 transmission is efficient or sporadic, Feng said.

（中国日报网英语点津 Helen 编辑）

About the broadcaster:

Emily Cheng is an editor at China Daily. She was born in Sydney, Australia and graduated from the University of Sydney with a degree in Media, English Literature and Politics. She has worked in the media industry since starting university and this is the third time she has settled abroad - she interned with a magazine in Hong Kong 2007 and studied at the University of Leeds in 2009.