If and when a pandemic of H1N1 swine flu hits, vaccines might be the world's best hope for softening the blow. But major uncertainties cloud the prospects for vaccines against the new strain. No pandemic vaccine yet exists, and it is unclear how much vaccine would have to be available, and by what time, to have any impact. Should manufacturers halt the production of seasonal influenza vaccine to focus on a pandemic version, and if so, when? And is there any way to ensure that people around the world have an equal chance to get the new vaccine?