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Researchers who combined a human flu virus with the H5N1 bird flu virus say that none of the hybrid viruses produced were capable of triggering a pandemic.

Their study, which was carried out in ferrets, is published online this week by Proceedings of the National Academy of Sciences.

So far H5N1 has proven mostly incapable of human-to-human transmission. In just a few cases, circumstances suggested that the virus might have spread between people, most notably in an Indonesian family this year (see No sign of bird flu mutation in Indonesian family).

Researchers have previously suggested that if the H5N1 virus were to combine with a human flu virus, it could become capable of easily infecting and spreading between people.

One study suggested that two proteins on the surface of H5N1 allow it to attach to and infect bird cells, but are ineffective at attaching to human cells (see H5N1 — why it can't spread between people). This led researchers to suggest that H5N1 might need surface proteins more like those on human flu to trigger a pandemic.

In this week's study, Jacqueline Katz of the US Centers for Disease Control and Prevention and her colleagues created various combinations of the H3N2 human flu virus and H5N1 viruses from Hong Kong, Indonesia and Vietnam.

Katz's team tested the viruses' ability to replicate and transmit in ferrets, whose respiratory tract cells interact with flu viruses similarly to those in humans.

One virus had human flu proteins on its outer surface, but bird flu genes inside. Although it was good at replicating, it was poor at spreading from ferret to ferret.

A virus with H5N1 external proteins and internal human-flu genes replicated as much as normal human flu virus but could not spread between ferrets at all.

The authors say that while the amount of virus a ferret produces in its nasal cavities may affect the chance of the virus being spread to a nearby animal, other factors in addition to the ability to trigger sneezes could be more important.

In the study, ferrets with human flu virus or viruses that had external proteins belonging to the human virus sneezed consistently.

Ferrets with any virus with H5N1's external proteins, however, sneezed rarely or not at all.

The authors point out that their study did not look at all possible combinations of human and H5N1 viruses.

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