In the early stages of the epidemic, before there has been significant depletion of susceptibles, the predicted vaccination strategy initially targets the 5 -14 and 15-24 year old age groups; vaccination should then begin in the 25 -44 age group, with older ages (greater than 45) and the younger ages (under 5) not being targeted until vaccine coverage exceeds 50 per cent. What is somewhat counterintuitive is that the optimal deployment of vaccine could be partial in many age classes. For example, if 50 per cent of the population can be vaccinated, then the optimal strategy would be to attain highest levels of coverage in the 25-44 year old age group and less in the ages 5 -24, despite the fact that the 5 -24 year old age groups are favoured for early vaccination before the 25-44 year old age group. A second feature emerges as vaccination is begun later in the epidemic. Because the epidemic process has already depleted much of the susceptible population in the most epidemiologically active age classes (namely the 5 -14 year olds and 15-24 year olds), there is a decreased benefit from large-scale vaccination of these age groups. It should be stressed that both the distribution of optimal vaccination at a given time, and the way this changes as the epidemic progresses, are critically dependent on the number of individuals in each age group, the mixing matrix (b a,b ) and hence the age-dependent susceptibility (q a ). This means that precise details will depend on the detailed epidemiology of the infection under investigation, and are therefore likely to vary between different strains of influenza-for example, with models parametrized to match the 2009 pandemic in England (see electronic supplementary material), the 1 -4 year old age group plays a far more dominant role.