The H7N9 bird flu virus claimed another life yesterday and five more people were confirmed to have contracted it.
That brings to 38 the number of cases confirmed by mainland authorities since the first on March 31, with 10 dead.
A four-year-old boy in Shanghai has been discharged after making a full recovery.
A 74-year-old retired man in Shanghai became the tenth death. He was diagnosed with pneumonia on Tuesday, confirmed to be infected with H7N9 on Wednesday and died yesterday, Shanghai authorities said.
An 83-year-old woman and 68-year-old man were also being treated and in stable condition.
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In Jiangsu, a 31-year-old Yangzhou cook and a 56-year-old Suzhou teacher were in critical condition, provincial health officials said.
Scientists and public health experts have been racing against the clock to study the virus, which has jumped for the first time from birds to humans.
A gene mutation known to help resist Tamiflu, one of the drugs recommended for treating H7N9, was found in the first of three H7N9 specimens from a Shanghai patient. The mutation, known as R292K, causes high-level resistance to the Roche-made drug and reduced sensitivity to a related drug from GlaxoSmithKline, Relenza, also recommended for treating H7N9 patients, according to information posted on the website of the Global Initiative on Sharing Avian Influenza Data.
Other H7N9 specimens from a patient in Shanghai and one in Anhui province do not show the mutation.
The finding warranted further analysis, a director at Japan's National Institute of Infectious Diseases in Tokyo, Masato Tashiro, said.
However, the World Health Organisation issued a statement saying preliminary tests had so far found no evidence that the H7N9 bird flu strain had developed resistance to Tamiflu and Relenza.
Tashiro said: "When you look at the raw data and compare the three strains of the virus, there's a signal from one strain that is less sensitive to both of the neuraminidase inhibitors (Tamiflu and Relenza)." It was "not a strong signal, but there's a possibility" of resistance.
View H7N9 map in a larger map
Click on each balloon for more information on individual patients infected with the avian flu virus: blue, patients infected with the H7N9 virus under treatment; red, those infected with the H7N9 who have died; and pink, those with H1N1 avian flu virus.