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

"We need to gather advice on priority groups for initial protection," WHO head Margaret Chan said Friday.

"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 to best distribute vaccines that will not be available in most cases before early October, at best, 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 hundreds of thousands and claiming at least 1,800 lives.

The WHO does suggest that health care workers should be given priority, a policy embraced by most states, but stops short of making further recommendations.

"Individual countries have to look at their own conditions and adapt," WHO spokesman Melinda Henry told AFP from Geneva.

"They have to decide whether they want to stop transmission, protect essential infrastructure, or reduce illness and death."

The problem is that these strategies each target a different segment of society, which means -- in a context of shortage -- that there simply won't be enough vaccine to protect everyone during the first critical months.

And that's not all: there is also disagreement among epidemiologists, who study how infections spread, as to which approach would save the most lives.

Researchers writing this week in the journal Science argue that 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 sharply at odds not only with the one adopted by most countries for fighting seasonal flu, but the one taking shape for the 2009 pandemic as well: prioritising what have long been identified as high-risk groups.

These usually include very young children older than six months, pregnant women, persons with chronic lung conditions, and the very elderly.

But pandemics don't behave like seasonal flu outbreaks, and don't always attack the same targets, experts say.

"The shift in mortality toward younger age groups was the most striking characteristic of the 20th-century pandemics," notes Mark Miller, a researcher at the Fogarty International Center, part of the US National Institutes of Health.

Early indications suggest the current pandemic -- while far less deadly than earlier ones, at least so far -- fits this pattern.

"With this virus the severe outcomes happen to people between 20 and 50 a lot of the time, which is very uncharacteristic of (seasonal) influenza," said Lone Simonsen, an epidemiologist at George Washington University and one of the architects of the US Influenza Genome Sequencing Project.

"There are some unusual risk factors too, such as severe obesity," she told AFP. Pregnant women seem to be at greater risk in all cases, she added.

And data from Mexico, the epicentre of the pandemic, shows that persons born before 1957 seems to have in-built immunity to the new strain, which could mean that the elderly, as a group, are less rather than more vulnerable.

Even tallying up potential lives lost is not a straight-forward process.

Are the deaths of a small child and a septuagenarian simply two lives lost, or does the child's weigh more heavily because of the greater number of "life years lost"?

"Which strategy countries choose is ultimately a political and ethical decision," the European Centre for Disease Prevention and Control said in a statement last year, before the current pandemic struck.

earlier related report
WHO urges China to share swine flu vaccine
With a likely "explosion" of swine flu in the coming months, a leading WHO official has urged China to distribute vaccines to needy nations to help contain the disease.

"We would greatly welcome an initiative by the Chinese government to support developing and needy countries in a fair distribution of this vaccine," Shin Young-soo, WHO's regional director for the Western Pacific, said in a statement received late Friday.

Shin's remarks came after Chinese Health Minister Chen Zhu reported on the nation's efforts to develop a vaccine for the A(H1N1) virus at a three-day symposium on swine flu in the Asia-Pacific region.

Shin warned the virus was entering an "acceleration period" and predicted more deaths and more cases as the "pandemic will get worse before the situation gets better."

"Most countries may see a doubling of cases every three to four days for two months until peak transmission is reached," Shin told the symposium Friday.

"At a certain point, there will seem to be an explosion in case numbers. I believe it is very likely that all countries will see community-level transmission by the end of the year."

About 1,800 people have died since the A(H1N1) virus was first uncovered in April, according to the latest update from the WHO issued this week. The vast majority of those deaths have been recorded in the Americas.

The WHO declared a global pandemic in June, the first such worldwide outbreak in 40 years. The UN health agency now says there are confirmed cases in more than 170 countries.

The WHO said earlier this week that countries in the northern hemisphere alone had ordered more than one billion doses of swine flu vaccine, sparking warnings about shortages, given the intense demand and production delays.

A Chinese drug company, Sinovac Biotech, this week announced positive preliminary data from its clinical trials after giving test subjects just one dose of its vaccine.

"We are also very encouraged by preliminary but promising results from the clinical trials of H1N1 vaccine," Shin said of the Chinese vaccine.

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