China's top health authority confirmed that a family infected by H7N9 in Shanghai might involve human-to-human transmission of the new bird flu strain.

The family involves two brothers and their 87-year-old father, who died on March 4 and was reportedly China's first human death from H7N9.

The first H7N9 bird flu patient in Beijing, a girl surnamed Yao, accompanied by her parents, receives a certificate from Ditan Hospital for her brave composure during her hospitalization there. Yao was discharged on Wednesday. Wang Jing / China Daily

The elder son, who has recovered from the disease, was previously confirmed to have contracted the virus, the Chinese National Health and Family Planning Commission said on Wednesday at a media briefing.

However, the results of a test on the younger son were not available.

"Further investigations are still under way to figure out whether the family cluster involved human-to-human transmission," said Feng Zijian, director of the health emergency center of the Chinese Center for Disease Control and Prevention.

Previous reports said the father had not been outside his home during the two weeks before the onset of his disease, which was five days after one or both of his sons were hospitalized with pneumonia symptoms.

Premier Li Keqiang vowed at a State Council meeting on Wednesday to use nationwide medical resources to cope with the flu.

The virus mainly jumps from birds to humans, and "human-to-human transmission, in theory, is possible, but is highly sporadic", Feng said.

Bird flu viruses, including the new strain of H7N9, do not easily infect humans, he added. "Notably, long-term and unprotected exposure to the infected person might result in another human infection."

Zeng Guang, chief epidemiologist with China CDC, said people infected with H7N9 can transmit the virus within a period of time, in which they could possibly infect others.

"But that's highly rare and could be limited to within a family," Feng said, explaining that only genetically vulnerable groups like close family members might be get infected.

The ability of the virus to spread among people is very weak, so transmission cannot spread beyond one person, he added. That belief is based on experience and studies with other bird flu strains such as H5N1.

"People don't need to panic, because such limited human-to-human transmission won't prompt a pandemic," he said.

Michael O'Leary, the World Health Organization's China representative, agreed and said there is no evidence to date of sustained or efficient human-to-human transmission, which would pose a real risk of a pandemic.

Feng said the government has been drafting plans for a pandemic response, and has initiated research and development of vaccines against H7N9.

As of Wednesday, China has reported 82 human infections of H7N9, including 17 deaths.

About 40 percent of the patients had no contact with poultry or environments where birds were located, previous epidemiological studies found. It remains a mystery how they became infected, Zeng said.

However, Feng raised questions over the accuracy of information collected.

"Only half of the sufferers could tell clearly if they were exposed to fowl or related environments," he said.