Governments bracing for a second, possibly more lethal, wave of swine flu are all grappling with the same unforgiving dilemma: with not enough vaccine to go around, who is going to get jabbed first?
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: prioritizing 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 Center for Disease Prevention and Control said in a statement last year, before the current pandemic struck.