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WHO: No sign of 'sustained' bird flu spread between humans

The CDC is developing a vaccine for a new strain of bird flu that has already killed at least five people in China. So far, the strain, known as H7N9, has not shown evidence that it can be passed person-to-person.

The World Health Organization said on Friday there was no sign of "sustained human-to-human transmission" of the H7N9 virus in China, but it was important to check on 400 people who had been in close contact with the 14 confirmed cases.

"We have 14 cases in a large geographical area, we have no sign of any epidemiological linkage between the confirmed cases and we have no sign of sustained human-to-human transmission," WHO spokesman Gregory Hartl told a news briefing in Geneva.

"The 400 contacts are being followed up to see if any of them do have the virus, have had it from someone else," he said.

"There are reports of people or a person with fever, so this is obviously why it's so important to follow up with all contacts in order to know whether or not they do have the virus and/or from whom they contracted it."

He added: "Remember even that if they are infected, you still need to try to find out if they contracted the virus from one another, or from a common environmental source."

Chinese authorities slaughtered over 20,000 birds on Friday at a poultry market in the financial hub Shanghai as the death toll from the new strain of bird flu mounted to six, spreading concern overseas and sparking a sell-off on Hong Kong's share market.

"It is really a severe illness but cases are being well handled and put into intensive care units. There doesn't seem to be any indication of infections in hospital so far," Hartl later told a group of reporters.