Drift opportunity in scenario C.

<p>Drift opportunity for scenario C with targeted distribution of a vaccine stockpile sufficient for 50 per cent coverage and assuming that pre-vaccination <i>R</i>_eff = 2. The axis labelled <i>ϵ</i>_v indicates the relative infectiousness of vaccinated and unvaccinated naïve hosts and “exp coverage” indicates the fractional vaccine coverage in experienced hosts (vaccine distribution is random when exp coverage is 0.5). Each panel corresponds to a different assumption regarding the form for the per infection rate of antigenic emergence <i>g</i>_0,<i>i</i>; equal probability per infection (<i>left</i>), proportional to host infectiousness (<i>middle</i>), and zero for vaccinated hosts with existing CTL protection (<i>right</i>). The vertical axis show the probability of emergence <i>P</i>_emg of an antigenically novel variant over the course of the epidemic relative to the same scenario without intervention.