The H7N9 bird flu virus is less likely to cause disease than the H5N1 virus that has killed hundreds of people worldwide, scientists say.

But tracking the new virus is harder, as it seems to spread quietly among poultry.

While the spread of H5N1 has been halted by culls of chickens, this will not work with the H7N9 virus, as it is not known to have caused widespread deaths among birds or other animals, says Professor Yuen Kwok-yung, a microbiologist at the University of Hong Kong.

Though symptoms are mild in animals infected with H7N9, the virus seems to be more deadly to humans, having killed three of the nine people infected so far and left the others critically ill.

"These avian viruses are not well adapted to humans, so they cause much more of a problem," Yuen said.

These avian viruses are not well adapted to humans, so they cause much more of a problem

Professor Yuen Kwok-yung, a microbiologist at the University of Hong Kong

Since culling is ineffective, the remaining preventive measures are screening tests and vaccination. Some Western countries have developed a vaccine against the H7 virus but its efficacy on H7N9 is unknown.

Scientists around the world have been seeking clues from the genome of the virus since it was obtained from the first three cases in Shanghai and published online on Sunday.

Analysis suggests that reassortment - in which different virus strains swap genes with one another in a host - gave birth to the new strain, according to an article published in science journal Nature on Tuesday. It appears to stem from the reassortment of three virus strains that only infect birds.

The H7N9 virus most likely originated in eastern China in wild birds, and was then transmitted to poultry and later to people, Yuan says.

Dr Masato Tashiro, who researches the genome of the virus at the World Health Organisation, says it has mutations that adapted to infect mammals. As such, pigs are also a possible infecting agent.

A feature of the virus is that its H protein is structurally similar to that of viruses that do not make birds severely ill. It has acquired key mutations that enable the H protein to latch onto receptors on mammal cells in the airways instead of bird receptors, according to the Nature article.

Yuen found that one of its eight gene segments had a mutation that helped it adapt to the human body temperature of 37 degrees Celsius.

As for how H7N9 infections will develop, Yuen says human infections may disappear suddenly, sporadic cases may continue before the virus returns as a more severe strain next winter, or human-to-human transmission may occur.

This article appeared in the South China Morning Post print edition as H7N9 harder to track but less likely to cause disease

"...having killed three of the nine people infected so far and left the others critically ill." One wonders how they know how many humans might have been infected with this if those people didn't become critically ill and seek treatment. The conclusion of only nine people being infected seems unwarranted from the facts presented.