The research looked at the structure of the flu neuraminidase (N1) which is the target for both Tamiflu and Relenza, the two existing flu drugs. Both drugs aim to inhibit the N1 which is responsible for the release of the virus from infected human cells and thus allows the disease to spread.

Using a method called X-ray crystallography the scientists looked at a mutation in the structure of N1 neuraminidase that has been observed in human cases of H5N1 and in seasonal flu. They found that when this mutation occurred, the virus became resistant to Tamiflu, while still remaining susceptible to Relenza.

Viruses have a high rate of mutation often adapting to the treatments devised to tackle them. It had previously been thought that this mutation in N1 made it less virulent, but recent research from the United States has shown that the mutation does not reduce the infectiousness of the virus.

The team led by Dr Steven Gamblin also looked at samples from the seasonal flu H1N1 and found that samples showing this mutation were also resistant to Tamiflu. While the proportion of seasonal flu samples showing this resistance varies widely across Europe and is relatively low in the UK, there is no telling how the seasonal virus will evolve next year.

Dr Steve Gamblin said: "What this research shows is that stockpiling any one drug to prepare for a potential H5N1 pandemic is unlikely to provide adequate cover. In order not to be outflanked by the virus, it will be necessary to have stocks of both existing drugs. We understand this is something the Government is already exploring. There is also a huge imperative to develop further drugs that could help disable this protein on the virus surface. It is likely a future pandemic will need to be tackled using a three- or four-pronged approach, much as we tackle HIV today."

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"Our results show that the H9N2 backbone vaccine can be used to protect mice against two different, highly pathogenic strains of influenza. We chose genes from H9N2 influenza for the vaccine because the virus can infect many different animals, including chickens, mice and pigs," said Professor Perez. "A very important limitation in the current design of flu vaccines is that they are usually species specific. Our approach involves a universal backbone that can be used in several different species, including humans."

More importantly, this live attenuated virus provided effective protection when it was administered to birds before they had hatched. By vaccinating eggs against influenza, we could protect wild bird species as well as domestic chickens against pandemic flu strains, limiting the spread of disease to humans.

"If an emerging strain of bird flu spreads among a broad range of animal species, we should expect major health, economic and ecological consequences," said Professor Perez. "It is unrealistic to consider preparing different vaccines specifically tailored to different animal species in this situation. An influenza vaccine that could protect different species would save valuable time during a pandemic."

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