Frequent travelers and rate of spread of epidemics

Abstract

A small proportion of air travelers make disproportionately more journeys than the rest of travelers. They also tend to interact predominantly with other frequent travelers in hotels and airport lounges. This group has the potential to accelerate global spread of infectious respiratory diseases. Using an epidemiologic model, we simulated exportation of cases from severe acute respiratory syndrome-like and influenza-like epidemics in a population for which a small proportion travel more frequently than the rest. Our simulations show that frequent travelers accelerate international spread of epidemics only if they are infected early in an outbreak and the outbreak does not expand rapidly. If the epidemic growth rate is high, as is likely for pandemic influenza, heterogeneities in travel are frequently overwhelmed by the large number of infected persons in the majority population and the resulting high probability that some of these persons will take an international flight.

Figure 1

Schematic representation of the model structure. Black boxes represent infectious stages and arrows indicate that persons in these populations are allowed to fly. A) Severe acute respiratory syndrome. Persons with latent infections are not infectious, and all infectious persons are symptomatic and prevented from traveling. B) Pandemic influenza. Persons with latent infections are infectious, and a proportion (1 – s) of infectious persons are asymptomatic and allowed to travel (indicated by the dotted arrows). The size of the arrows indicates that the persons in the high-frequency flier group have a higher probability of flying per day.

Figure 2

Mean number of cases exported from a single simulated source epidemic for severe acute respiratory syndrome–like parameters (A) and influenza-like parameters (B) (50,000 runs; parameters are listed in Table 1). Results are shown for a population in which everyone travels equally frequently (homogeneous model, circles), for a population in which 1% travel 20 times more frequently than the rest of the population, and for the 2 populations mixed randomly (Φ = 1, squares) for moderate levels of mixing between the groups (Φ = 0.5, diamonds) and for low levels of mixing in which most contacts are assortative (Φ = 0.25, triangles). The first cases are either in the majority population of low-frequency fliers (solid symbols) or high-frequency fliers (open symbols). Inset in B shows a greater range on the y-axis. Variability about these means is shown in Table 2 and Appendix Figure 1.