Two people in the central Chinese province of Henan have been infected by a new strain of avian influenza, the first cases found in the region, while the death toll has risen to 13 from a total of 60 infections after two more deaths in Shanghai.
One of the Henan victims, a 34-year old man in the city of Kaifeng, is now critically ill in hospital, while the other, a 65-year old farmer from Zhoukou, is stable. The two cases do not appear to be connected.
A total of 19 people in close contact with the two new victims were under observation but had shown no signs of infection, state news agency Xinhua said.
Another four cases have been confirmed in eastern Zhejiang, Xinhua said on Sunday, bringing the total number in the province to 15. None of the 483 people in close contact with the victims has presented any symptoms.
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Three more victims were identified in Shanghai, China's business hub, bringing the total number of cases in the city to 24, with a total of nine deaths, state media said.
Three cases have now been reported outside the original clusters in eastern China, including one in the capital Beijing, but there is nothing out of the ordinary so far, the China representative of the World Health Organization said.
"There's no way to predict how it'll spread but it's not surprising if we have new cases in different places like we do in Beijing," Michael O'Leary told reporters.
On Saturday, the China Centre for Disease Control and Prevention confirmed that a seven year-old child in the capital of Beijing had been infected by the H7N9 bird flu virus, the first case to be reported outside of the Yangtze river delta region in east China, where the new strain emerged last month.
The child's parents work in the poultry trade.
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Investigators are trying to ascertain the source amid fears that it could cause a deadly pandemic similar to Severe Acute Respiratory Syndrome (SARS) in 2003, which killed about one in 10 of the 8,000 people it infected worldwide.
China has been anxious to avoid a repeat of the panic of 2003 by promising total transparency, and O'Leary said his organisation has been "very pleased" about the way information was being shared.
China's health ministry said on Saturday that there is still no indication of human-to-human transmission of the virus, which has killed 13 people in Shanghai and the provinces of Zhejiang, Jiangsu and Anhui.
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"That's a key factor in this situation," said O'Leary. "As far as we know, all the cases are individually infected in a sporadic and not connected way."
The husband of a H7N9 victim in Shanghai was recently infected, but O'Leary said there was no cause for alarm.
"If there's only very rare cases ... That's different from the ease of transmission from person to person. It's that ease of transmission that we are concerned about, and there's no evidence of that yet."