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).
WHO head Margaret Chan said: "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 WHO does suggest that health care workers should be given priority, a policy embraced by most states, but stops short of making further recommendations.
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.
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 pregnant women, very young children older than six months, 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.