As scientists delve deeper into H5N1 bird flu, they have discovered it is only three steps away from mutating into a potentially lethal human pandemic form.

The find has added new urgency to scientists' quest for a way to prevent a deadly global outbreak that could kill millions.

Culls of hundreds of thousands of chickens, turkeys and ducks to stem bird flu outbreaks rarely make international headlines these days, but they are a worryingly common event as the deadly virus continues its march across the globe.

In 2009, during the H1N1 swine flu pandemic, vaccines only became available months after the virus had spread around the world - and even then there was only enough for one in five of the world's 7 billion people.

Next time, experts say, we need another approach.

Talk is centred on ‘pre-pandemic vaccination’ - immunising people years in advance against a flu pandemic that has yet to happen, and may never come, rather than rushing to create vaccines once a new pandemic starts.