New publication on human mobility restrictions measures against the global spreading of H1N1pdm influenza

Travel-related and mobility restriction policies are often considered to be the first line of defence against the spread of an emerging infectious disease. The wide range of implementation scenarios of such policies calls for models that are able to anticipate their actual effectiveness. The GLEaM team tackles this problem in the recent publication:

Human mobility networks, travel restrictions and the global spread of 2009 H1N1 pandemic
Paolo Bajardi, Chiara Poletto, Jose J Ramasco, Michele Tizzoni, Vittoria Colizza, Alessandro Vespignani
PLoS ONE 6(1): e16591 (2011).

During the early phase of the 2009 H1N1 pandemic outbreak, some countries implemented travel-related measures to prevent the infection from crossing the national borders. Many governments advised against non-essential travel to Mexico and activated airports entry screening to detect the potentially infected travelers. Even a few countries banned every flight connection to/from Mexico. All these measures, with the addition of self-imposed travel limitations due to the pandemic concerns following the international alert, contributed to an almost 40% reduction in the international passengers flying to and from Mexico. However, no containment was achieved by such restrictions and the virus was able to reach pandemic proportions in a short time.

Illustration of the global invasion of the 2009 H1N1 pandemic during the early stage of the outbreak. The arrows represent the seeding of unaffected countries due to infected individuals traveling from Mexico. The color code indicates the time of the seeding. The map shows the layer of the worldwide air transportation network, which is incorporated into GLEaM.

Illustration of the global invasion of the 2009 H1N1 pandemic during the early stage of the outbreak. The arrows represent the seeding of unaffected countries due to infected individuals traveling from Mexico. The color code indicates the time of the seeding. The map shows the layer of the worldwide air transportation network, which is incorporated into GLEaM.

GLEaM is suitable to simulate the spreading of an influenza-like illness and, in particular, it has been calibrated to simulate the 2009 H1N1 pandemic considering the etiology of the disease and the initial conditions. Taking advantage of the high detailed mobility data at the global level integrated in the model structure, Bajardi and coworkers assessed the impact of different travel reduction policies in the unfolding of the simulated pandemics. The work  shows that feasible mobility limitations, highly disruptive in economic terms, generally are not effective: even with strong and lasting restrictions (a 90% reduction in the international air traffic to/from Mexico starting with the international alert and kept to the end of the epidemic was tested) the delay achieved is limited to two weeks .

A, Delay in the case importation from Mexico to a given country compared with the reference scenario. The parameter α is a worldwide passengers reduction to/from Mexico. The computational approach allows tests on draconian restrictions like α=90%. The dotted line shows the logarithmic behavior relating the delay as a function of the imposed restrictions. B, The global invasion threshold R  is the key parameter for describing a global outbreak: if R  >1, the disease might globally spread. The basic reproductive number R0 is the average number of infections that a typical infectious individual can generate. Only in the case of extremely low values of R0 or extremely large values of α is it possible to reduce R  below the threshold.

A) Delay in the case importation from Mexico to a given country compared with the reference scenario. The parameter α is a worldwide passengers reduction to/from Mexico. The computational approach allows tests on draconian restrictions like α=90%. The dotted line shows the logarithmic behavior relating the delay as a function of the imposed restrictions. B) The global invasion threshold R is the key parameter for describing a global outbreak: if R* >1, the disease might globally spread. The basic reproductive number R0 is the average number of infections that a typical infectious individual can generate. Only in the case of extremely low values of R0 or extremely large values of α is it possible to reduce R* below the threshold.

In a pandemic scenario, this delay can be used to allocate resources and to enhance the surveillance systems, but it is definitely too short to develop a vaccine. Finally, the paper provides a quantitative discussion devoted to explain how the large heterogeneity of human mobility patterns is responsible for the ineffectiveness of travel restrictions. It is unlikely that, given the ever-increasing mobility of people around the world, travel restrictions could be used effectively in a future pandemic event.

Other News