We now consider the optimal prioritization (either group D first or group H first), as four key parameters are varied: the transmission rate within the dominant transmitter group, b DD ; the relative adverse consequences of infection for the three groups, s H . s D ¼ s G ; the timing for the start of the vaccination programme, T; and the speed with which the population is vaccinated, v. Here the consequences of infection could capture a variety of measures, from risk of symptoms if infected, to concepts such as loss of QALYs (auality-adjusted life years), to risk of hospitalization, to risk of mortality associated with infection. The curves shown in figure 2 separate regions of parameter space where one form of prioritization is optimal, in terms of minimizing the total consequences of infection over the entire epidemic and across all three groups; regions above and to the left of the curves are where it is best to initially target vaccination towards the group with potentially severe health complications.