Any lingering hopes that pharmaceutical companies could rapidly fill orders for more than a billion doses from northern hemisphere countries alone were quashed last week by the World Health Organisation.

The organisation's head, Margaret Chan, said, ''We need to gather advice on priority groups for initial protection.''

''This is one of the most difficult decisions governments around the world will need to make, especially as we know that supplies will be extremely limited for some months to come.''

But national leaders looking for guidance from international health authorities on how best to distribute vaccines are bound to be disappointed. The European Union has yet to issue any guidelines specific to the new strain of A(H1N1) influenza that has swept across the globe, infecting thousands of people and claiming at least 1800 lives.

The WHO suggests health-care workers should be given priority, but stops short of making further recommendations.

A WHO spokeswoman Melinda Henry said countries ''have to decide whether they want to stop transmission, protect essential infrastructure, or reduce illness and death''.

But these strategies each target a different segment of society which means, in the event of a shortage, there won't be enough vaccine to protect everyone during the first critical months.

There is also disagreement among epidemiologists as to which approach would save the most lives.

Researchers writing last week in the journal Science argue the best way to halt the spread of the virus for pandemic flu is to vaccinate school-age children and their parents first.

The study makes projections for the United States, but the model could be applicable in other developed nations, and perhaps across the board.

In Japan, the systematic vaccination of school children prevented more than 40,000 deaths across all age groups every year from the 1960s until the policy was dropped in 1994, according to an earlier study in the New England Journal of Medicine.

This strategy, however, is at odds with prioritising what has long been identified as high-risk groups.

These usually include young children, older than six months, pregnant women and the elderly. AFP