HONG KONG - Leading scientists in Hong Kong have found that the H5N1 bird flu virus can infect cells in the upper airway of humans and need not penetrate deep in the lungs to cause infection.

A study by scientists based in the United States in 2006 suggested that H5N1 could not infect people easily because it had to first lodge itself deep inside the lungs, where it binds more easily to certain receptors called the alpha 2-3.

But in an article published in the January issue of the journal Nature Medicine, scientists from the University of Hong Kong found that the virus could infect the nasopharynx, an area behind the nose and above the soft palate, and the throat.

"On the earlier hypothesis, the virus has to go deep into the lungs to infect anybody but our research suggests that is not the case. The virus can get a foothold in the upper respiratory tract, it doesn't have to get deep down into the lungs," microbiology professor Malik Peiris told Reuters late on Friday.

No need to panic
Using discarded human tissues, Malik found both upper and lower human respiratory tracts could be infected by the virus.

"Even in the upper respiratory tract (where) the alpha 2-3 receptor seems to be lacking, the H5N1 can still infect the cells ... so it raises the question of whether there may be other receptors the virus is using and highlights the point that further study is needed."

However, he said there was no reason to panic.

"It is still not able in most cases to establish infection and has not been able to transmit human to human (efficiently). It doesn't change that situation as such," said Peiris, who has studied the H5N1 since 1997, when it made its first known jump to humans in Hong Kong, killing six people.

The virus re-emerged in late 2003 and has become endemic in several places in Asia. It has since infected 270 people around the world, killing 164 of them, according to latest figures from the World Health Organisation.

It has flared up again in recent months, spreading through poultry flocks in Japan, Vietnam and Thailand, killing six people in Indonesia and claiming its first human life in Nigeria.

Although it remains a bird disease, experts still fear it could kill millions once it learns how to pass efficiently among people.