Bird Flu Pandemic May Be Less Likely

Researchers in a high-security lab infected ferrets with genetically engineered versions of H5N1 avian influenza and found that the animals did not infect other ferrets caged nearby. The hybrid flu versions also appeared less virulent than their parent strains.

Flu researchers and virologists expressed guarded optimism about the findings.

"The fact that a simple gene swap didn't immediately create a monster virus is reassuring," said Dr. Anna Moscona, an infectious disease specialist at Cornell University's Weill Medical College who was not connected with the research.

Still, Dr. Julie L. Gerberding, director of the Centers for Disease Control and Prevention, which led the research, sounded a cautious note about the study.

"These data do not mean that H5N1 cannot develop into a pandemic strain, it means that the genetics of that transformation are more complicated than a simple one-to-one exchange," she said.

"Though we weren't able to do this through some simple gene exchanges, there are many other combinations and subtle changes that the virus itself could make," she said.

The H5N1 avian influenza has remained mostly a disease of birds, although it has occasionally infected humans. According to the World Health Organization, 232 people have been infected since 2003, of whom 134 died.

Health officials worry that the disease could mutate into a form that could spread much more efficiently among people, leading to a pandemic.

Previous studies have shown that flu epidemics in 1957 and 1968 were caused by exchanges of genes between avian and human influenza viruses. CDC researchers tried to mimic this process, said Taronna Maines, a CDC microbiologist and lead author of the study.

Because they began the work in 2002, the researchers picked the only H5N1 strain known to cause human infection at that time -- a 1997 strain from Hong Kong. They chose a human H3N2 flu strain because it is responsible for the most annual flu infections.

Out of 50 possible variations, the researchers created three hybrid strains.

"We tested the combinations that we thought had a good chance of being able to replicate well both in culture and in the ferrets," said team leader Jackie Katz, a CDC microbiologist.