WHO / A. Crump

This week's announcement of a potential vaccine against the bird flu virus that many fear could kill millions in a global pandemic, was tempered by the news that it only appears to work at unfeasibly high doses.

The world's vaccine factories could simply not produce enough of it to prevent a pandemic, say flu experts.

Preliminary data suggests that people will need to be injected with 180 micrograms of the vaccine for it to be effective. This is 12 times the dose used to protect people from strains of ordinary flu.

Scientists say the findings show that they need to find ways to use the vaccine more sparingly and replace the chicken eggs currently used to produce it with more efficient systems.

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