Inflow restrictions can prevent epidemics when contact tracing efforts are effective but have limited capacity

Abstract

When a region tries to prevent an outbreak of an epidemic, two broad strategies are available: limiting the inflow of infected cases by using travel restrictions and quarantines or limiting the risk of local transmission from imported cases by using contact tracing and other community interventions. A number of papers have used epidemiological models to argue that inflow restrictions are unlikely to be effective. We simulate a simple epidemiological model to show that this conclusion changes if containment efforts such as contact tracing have limited capacity. In particular, our results show that moderate travel restrictions can lead to large reductions in the probability of an epidemic when contact tracing is effective but the contact tracing system is close to being overwhelmed.

Keywords: contact tracing; epidemics; travel restrictions.

Figure 1.

Simplified model of an epidemic outbreak with contact tracing. This diagram outlines the basic evolution of a disease from emergence to epidemic outbreak in the presence of a contact tracing system. When the system is imperfect, each traced case has a positive probability of leading to an epidemic, regardless of the arrival rate of new cases, the rate at which cases are processed or the number of cases that can be processed at once. When the system is effective, an outbreak will only occur if the system's capacity is limited and not all newly arriving cases can be processed.

Figure 2.

Model-simulated probability of an epidemic outbreak as a function of time. Each panel shows the results of model simulations estimating the probability of an epidemic occurring as a function of time given one of three possible arrival rates for new infected cases: a baseline arrival rate, a moderately reduced arrival rate and a strongly reduced arrival rate. Here (a,b) display results from simulations where contact tracing is assumed to have unlimited capacity, while (c,d) assume limited capacity. Here (a,c) display results from simulations where contract tracing is assumed to be less than fully effective, while (b,d) assume that it is fully effective.