But that behavior could change if the virus mutates. Experts say the new research doesn't indicate how likely the virus is to change genetically and unleash a worldwide outbreak of lethal flu. However, the work suggests one of the signs to watch for in new virus samples to help gauge the danger to humans.
The work, reported in Thursday's issue of the journal Nature, comes from University of Wisconsin-Madison virologist Yoshihiro Kawaoka with colleagues in Japan. Similar results, from the Erasmus Medical Center in Rotterdam, the Netherlands, will be published online Thursday by the journal Science.
More than 180 people are known to have been infected with the bird flu virus H5N1. Virtually all are believed to have caught it from infected poultry. But scientists have long warned that the virus, which is prone to mutation, could transform itself into a version that spreads easily from person to person. That germ could touch off a pandemic.
Bird flu has not yet reached North and South America, but scientists expect it to arrive soon along with birds migrating from Asia, outgoing Interior Secretary Gale Norton explained to CBS Evening News anchor Bob Schieffer.
However,that people should not be afraid to eat poultry, because cooking kills the virus.
Ordinary flu viruses spread when an infected person coughs or sneezes, blasting out tiny droplets carrying the germ. For that to happen, the virus has to be perched in the right places to be ejected by a cough or sneeze. The new work suggests H5N1, by contrast, infects humans too low in the respiratory tract for that to occur.
Both research teams used human tissue removed from various parts of the respiratory tract — the region from the nose to the lung — to study where virus infection occurs.
Scientists already knew that bird flu viruses use a specific kind of docking site to enter cells they infect, while human flu viruses use a different one. Kawaoka's group found the bird virus docking site appears mostly on lung cells, while being rare on cells found in higher areas like the nose and windpipe. Those higher areas were dominated instead by the human-type docking site.
Kawaoka said that for H5N1 to become a pandemic virus, it would have to mutate in a way that lets it attach to the same docking site human viruses use. Other mutations would be needed as well, he said in a statement.
Robert M. Krug of the University of Texas at Austin called Kawaoka's work an important observation, and said that if H5N1 begins to use the human virus docking site "we've got a lot to worry about." It's not clear whether that would be enough to produce a pandemic germ, he said.
James Paulson of the Scripps Research Institute in La Jolla, Calif., stressed that other viral factors may be important in human-to-human transmission. But he said that once the virus has a foothold in a person, it may mutate to gain the abilities it needs to start spreading among people.