First H7N9 flu case in Taiwan(2)

Yang Weizhong, deputy director of the Chinese Center for Disease Control and Prevention, said: "We know very little about immunity against this virus and how this virus causes diseases. We are planning to develop vaccines against it. But my personal experience tells me it will be difficult and will take a long time to succeed."

Experts also said human infection of H7N9 is highly likely to have originated from live poultry markets.

"According to the evidence that we've got, birds infected by the virus, especially poultry, and the environmentcontaminated by the virus are the most likely sources of infection," said Fukuda from the WHO.

This hypothesis is supported by experts' observations in Shanghai, said Anne Kelso at the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne, Australia.

"On April 6, Shanghai rapidly closed down the live poultry markets. It's been very encouragingto see that almost immediately there was a decline in the detection of new cases. The cases that did occur all occurred in the next week (after the shutdown), which were within the incubation period of the virus," she said. "The evidence suggests that closing down live poultry markets is an effective way to reduce the risk of infection."

Shanghai confirmed its first case of a human infected with H7N9 on March 30, and had been reporting sporadic cases of infection almost every day since the beginning of April.

However, all the cases reported showed related symptoms before April 13. The city has not reported a newly diagnosed case for four consecutive days.

Other cities, including Nanjing and Hangzhou, also closed down their live poultry markets.

Kelso said it is important to see if there is a similar decline of new cases in these areas in the coming weeks.

   Special report:

    H7N9 avian influenza