The bird flu virus that spread from a son to his father showed no signs of human mutation, a spokesman for the Ministry of Health, Mao Qun'an, said on Sunday.

It showed no biological features for human-to-human transmission, he said.

Mao was commenting on an April 8 report by the British medical weekly, Lancet, that human-to-human transmission of the H5N1 virus had been discovered within one family.

Research into last December's case in Nanjing, Jiangsu Province was conducted by the Chinese Center for Disease Control and Prevention.

A 52-year-old man contracted the virus from close contact with his infected son, who died despite treatment. Both men had not come into contact with sick or dead poultry.

The World Health Organization defined the Nanjing case as "limited, non-sustained, person-to-person transmission", and dismissed speculation that the virus had mutated into a form easily transmitted between humans.

"Laboratory tests showed the virus detected in the body of the father and son was an animal-borne one that affects only birds, and not humans," Mao told China Daily.

"It was not normal social contact that led to the human transmission," British epidemiologist Jeremy Farrar said.

Besides the father, none of the 91 people who came into contact with his son showed any signs of infection, the Ministry of Health said.

To date, the majority of the 378 human cases of H5N1 bird flu diagnosed in the world since 2003, was spread by domestic or wild birds, the WHO said. More than 60 percent were fatal.

China has recorded 20 deaths out of 30 such cases, official statistics show.

(China Daily April 15, 2008)