Health authorities in eastern China said they had so far detected no human cases of bird flu after a woman in Hong Kong who had visited the region contracted the potentially fatal disease.
"Currently no cases have emerged," the Shanghai Health Bureau said in a statement.
It said experts had been dispatched to find people who came into close contact with the 59-year-old woman, who is in a serious condition in a Hong Kong hospital isolation ward.
The patient had recently visited the eastern Chinese cities of Nanjing, Shanghai and Hangzhou, but officials in Hong Kong said it was too early to say where or how she contracted the disease.
According to the Chinese statement dated Thursday, the woman's son, daughter-in-law and others including hotel employees in Shanghai had been checked and did not present symptoms of the disease.
Health workers would continue to monitor them and send them to hospital if they developed a fever, sore throat or other symptoms, it added. They were also monitoring people near the hotel where she stayed, such as food vendors.
The health bureau in Jiangsu province, where Nanjing is located, said it too had checked more than 30 hotel workers who came into contact with the patient, and that they had not developed symptoms of the disease.
Health authorities in Zhejiang province, where Hangzhou is located, and China's health and agriculture ministries were not immediately available for comment.
The re-emergence of the disease has caused concern in the teeming city of Hong Kong, where the last human case of bird flu was recorded in 2003.
In mainland China, however, the disease has re-occurred several times.
The latest reported human case was in June, when a young pregnant woman in the central province of Hubei died of bird flu, bringing to 26 the number of people killed by the virus since it re-emerged in 2003.