China's health authorities said on Monday that no human-to-human transmission had been confirmed in the two human cases of bird flu and the means of transmission in these cases, involving two family members in the Nanjing area, remained unknown.

"The virus in the first case originated with poultry and has not mutated. It has no biological features for human-to-human transmission," Mao Qun'an, spokesman of the Ministry of Health, said, citing laboratory results.

But Mao said there were several possibilities for transmission in the second human case, which involved the father of the man in first case.

Mao Qun'an, spokesman of the Ministry of Health speaks in press conference that no human-to-human transmission had been confirmed in the two human cases of bird flu in the Nanjing area.

He said epidemiological investigation showed that the father could have become infected through close contact with the son, but it was also possible that both men were either infected by same source or separately from different sources.

"Neither of the three possibilities has been confirmed and in-depth investigation is still being conducted," Mao said.

China confirmed on Dec. 7 that the father of a 24-year-old Chinese man who died of H5N1 infection on Dec. 2 had also been infected with the H5N1 virus that causes the disease.

The 52-year-old man, surnamed Lu, was a native of Nanjing, capital of Jiangsu. He developed fever and was hospitalized for lower lobe pneumonia on Dec. 3, according to the Ministry of Health.

Lu's son was said to have no known contact with dead poultry and the Jiangsu Animal Husbandry and Veterinary Bureau said earlier last week that no bird flu epidemic had been discovered in the province.

Mao said the father was in stable condition and showing signs of improvement.

Mao said everyone who had close contact with either man was under close observation.

He said among the 69 people who had close contact with the son, 55 had been released from medical observation. "Except the father, none of those people have shown unusual symptoms," he said.

Among the 20 close contacts with the father, six also had close contact with the son, Mao said. The six people were currently under strict medical observation, but none had shown unusual symptoms so far, he added.

The latest cases bring the number of confirmed human infections of bird flu in China to 27 since 2003, with 17 deaths.

Bird flu, or Avian influenza, is a contagious disease of animal origin caused by viruses that normally infect only birds and, less commonly, pigs.

The World Health Organization have been warning that the virus -- if given enough opportunities -- will mutate into a form that is highly infectious among humans and easily transmissible from person to person. Such a change could mark the start of a global outbreak.

Editor: Yan