Global invasion of H1N1 influenza: could have we stopped it by grounding planes?

 

PAOLO BAJARDI

ISI FOUNDATION

 

After the emergence of the H1N1 influenza in 2009, some countries responded with travel-related controls during the early stage of the outbreak, in the attempt to contain or slow down its international spread. Travel bans, airport screenings, travel advisories, along with self-imposed travel limitations, contributed to the observed decline of about 40% in the international air traffic to/from Mexico following the international alert. With limited quantitative evidence from real-world examples to test rigorously the value of travel-related measures, only simulation efforts are available that point to the scarce efficacy of flight bans and travel reductions in the context of pandemic planning. In order to provide a comprehensive computational and theoretical understanding of this phenomenon, we present a detailed analysis of the effect of travel restrictions on the course of the 2009 H1N1 pandemic influenza. Using a Global Epidemic and Mobility Model (GLEaM) that explicitly integrates air travel and short range mobility data, we evaluate the magnitude of impact of increasingly strict regimes of travel reductions in terms of changes in the seeding events and delay in the arrival times of the infection. Our in-silico experiments highlight the very limited value of international travel restrictions alone, which would not contain the pandemic, achieving only small delays in the seeding events even if strong and lasting. By introducing a theoretical framework describing the infection in a spatially structured system, these results are explained in terms of a phase transition in the individual mobility characterizing the invasion dynamics. The real-world air travel system is found to be largely above the critical mobility threshold, thus vanishing the potential role of mobility reductions aimed at containment.