Above we considered the impact of different interventions aimed at reducing infection transmission, considering each intervention in isolation. By looking at them separately we get a clear indication of how effective each intervention is, and a comparison of their separate effects shows their relative effectiveness. In practice, however, a combination of such interventions will be used and it is of interest to see how effective this may be. In particular, we might wish to know what combination of interventions can achieve disease elimination and whether such a combination can be achieved in practice.
Here we give illustrative calculations of the effect of some combinations of interventions. We use a model that distinguishes between three types of individual (general population members, general practitioners and influenza-dedicated health care workers). Further details of the model are given in Section 5 (see subsection 5.2), which deals in greater detail with the role of antiviral drugs. Figure 4.17 illustrates how, compared with the case of no intervention (when R0 obtains), the distribution of AVs to doctors, patients and health care workers, with PPE for the latter, impacts on the effective R. This strategy (Strategy A) might be motivated by the need to maintain a sustainable health service. It reduces the transmission of the infection marginally. Further, the figure illustrates the significant impact of Strategy B, which combines Strategy A with isolation (ƒ =0.75) and personal infection control and distancing (λS × λI =0.25). For values of R0 as high as about 5.5, this combination of interventions can reduce the effective R to below 1, thereby eliminating the infection.
Figure 4.17 The amount by which a combination of strategies changes R, as a function of R0.
Strategy A: AVs are distributed to doctors (post-exposure prophylaxis), to patients (treatment) and to influenza-dedicated health care workers (pre-exposure prophylaxis), with the health care workers also using PPE (the default strategy for antiviral use).
Strategy B: a combination of Strategy A, personal infection control and distancing (λS × λI =0.25) and isolation (spending 75% of infectivity in the community before being isolated, i.e. ƒ =0.75 ). Clearly, this combination reduces the effective R significantly.
School children are a special group of interest, as they have been found in some disease studies to be responsible for a disproportionate contribution of disease transmission through greater infectivity, less prior immunity and enhanced mixing in school environments. Thus a more general model is considered now, which includes a fourth class, namely school children, as well as a household structure. It includes prior immunity and enhanced school mixing, as well as a structure that allows a proportion of parents to stay home from work and care for children when schools are closed. Figure 4.18 illustrates the impact of a combination of such measures. Strategy A is, as in Figure 4.17, aims to maintain a sustainable health care service. Its effect is again shown by the heavy line. This strategy together with closing schools, giving AVs to all school children, as well as closing schools and distributing AVs to all school children as prophylaxis, are the combinations of interventions considered. Also included in this figure is the impact of closing non-essential workplaces, together with closing schools and the default antiviral distribution to protect health care workers. The results indicate that closing 50% of non-essential workplaces is more effective than providing AVs to all school children, when both strategies are in combination with the default antiviral strategy and the closure of schools.
While each combination of strategies has a significant impact on transmission, the results suggest that isolation and personal infection control and distancing, and the closure of schools and workplaces (a means of enforced social distancing) aresubstantially more effective than the others.
Figure 4.18 The impact on the effective R of closing schools, closing non-essential workplaces and providing school children with antiviral prophylaxis is illustrated. The first graph plots the curves for which the effective R = 1 for a parameter combination of ƒ (the proportion of infectivity spent in the community) and R0. Note that R > 1 above the curve so that a major epidemic may occur, while R < 1 below the curve so that the outbreak is contained. The second graph plots the effective R for given R0 and a particular strategy.
Strategy A: AVs are distributed to doctors, patients and health care workers, with the latter using PPE (the default strategy for antiviral use).
Strategy P: PPE (only) used for HCWs (no AVs distributed).
Strategy C: no intervention.
Strategy D: Strategy A together with closing schools and a proportion of parents remaining home to care for them.
Strategy E: Strategy A together with all school children receiving AVs as prophylaxis.
Strategy F: Strategy D together with all school children receiving AVs as prophylaxis.
Strategies G and H: Strategy A together with closing schools and 50% and 100%, respectively, of all working adults staying at home.