No bird flu virus was found in dead pig samples from a river that provides drinking water to residents in Shanghai where two died in the first human infections of a new avian influenza strain, authorities said Monday.

Two people in Shanghai, 87 and 27 years old, became ill with fever and coughs in late February and died in early March, suffering from severe pneumonia and difficult breathing, the National Health and Family Planning Commission said Sunday.

The two have been confirmed to be infected with H7N9 avian influenza by an expert team summoned by the health and family planning commission, based on clinical observation, laboratory tests and epidemiological surveys.

On Monday, the Shanghai Animal Disease Prevention and Control Center tested 34 samples of pig carcasses pulled from the Huangpu River running through the city and found no bird flu viruses, the city government said in a statement.

Thousands of dead pigs have been retrieved from the Huangpu River last month, sparking huge panic as well as satire among the public over tap water safety.

The city's health authorities will beef up monitoring over cases of influenza and pneumonia caused by unknown reasons, according to the statement.

In the first quarter of the year, the city reported similar number of influenza and pneumonia cases compared with average levels over the past three years, it said.

Meanwhile, more than 100 monitoring sites and hospitals in the city reported no cases of SARS and H5N1 avian influenza from mid November to end March, it said.

A third person, a 35-year-old female in Chuzhou City of Anhui Province became ill on March 9 and was now in a critical condition after contracting the H7N9 strain.

It is unclear how the three got infected, and no mutual infections were discovered among them, said the National Health and Family Planning Commission. Besides, no abnormalities were detected among 88 of their close contacts.

The subtype of H7N9 bird flu virus has not been contracted to human beings before. The virus shows no signs of being highly contagious among humans, according to the clinical observation on the cases' close contacts.

However, as only three cases of human infection of H7N9 have been found, relatively little research has been done on it. The expert team is working to study the toxicity and human-infection capacity of the virus, according to the commission.

There are no vaccines against the H7N9 bird flu virus either at home or abroad.