PARIS (AFP) - A 24-year old man in China probably infected his father with the H5N1 strain of bird flu before dying, renewing concerns that the disease could one day spread easily among humans, according to a study released Tuesday.

The case is one of a handful over the last four years in which the H5N1 virus is suspected to have spread from one person to another, according to lead researcher Yu Wang of the Chinese Centre for Disease Control.

To date, however, all such cases have been what scientists call "limited, non-sustained, person-to-person transmission," meaning that contagion only occurs under very specific circumstances.

The vast majority of the known 378 human cases of H5N1 bird flu since 2003 were spread by domestic or wild fowl, according to the World Health Organisation. More than 60 percent proved fatal.

"It is not normal social contact that has led to the human transmission," epidemiologist Jeremy Farrar, a researcher at the national Institute of Hygiene and Epidemiology, told AFP in an interview.

"In this case it took extensive exposure to secretions of somebody who was very sick in hospital," he explained.

Another limiting factor may be genetic, the study found. The suspected cases of human-to-human transmission have all been "within the family, among blood relatives," said Farrar.

None of the 91 persons besides the father who came into contact with his son before he died showed any sign of infection, said the study, published in the British medical weekly The Lancet.

Nor was there any significant genetic variation between the viral strain in the father or the son.

Experts fear that the H5N1 virus could mutate after infecting one human into a more contagious form, as occurred during at least three flu pandemics in the 20th century.

An estimated 20 to 40 million people perished in the so-called "Spanish flu" of 1918.

Any new clusters of the virus "require urgent investigation because of the possibility that a change in the epidemiology of H5N1 cases could indicate that H5N1 viruses have acquired the ability to spread more easily among people," said Wang.

The two cases examined in the study were identified in December in the city of Nanjing, in China's Jiangsu Province.

Since 2003, there have been 107 H5N1 bird flu fatalities in Indonesia, 52 in Vietnam, 20 in China, 17 in Thailand, and between one and seven in seven other nations.