Small islands and pandemic influenza: Potential benefits and limitations of travel volume reduction as a border control measure

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Abstract
Some island nations have explicit components of their influenza pandemic plans for providing travel warnings and restricting incoming travellers. But the potential value of such restrictions has not been quantified. We developed a probabilistic model and used parameters from a published model (i.e., InfluSim) and travel data from Pacific Island Countries and Territories (PICTs). The results indicate that of the 17 PICTs with travel data, only six would be likely to escape a major pandemic with a viral strain of relatively low contagiousness (i.e., for R 0 = 1.5) even when imposing very tight travel volume reductions of 99% throughout the course of the pandemic. For a more contagious viral strain (R 0 = 2.25) only five PICTs would have a probability of over 50% to escape. The total number of travellers during the pandemic must not exceed 115 (for R 0 = 3.0) or 380 (for R 0 = 1.5) if a PICT aims to keep the probability of pandemic arrival below 50%. These results suggest that relatively few island nations could successfully rely on intensive travel volume restrictions alone to avoid the arrival of pandemic influenza (or subsequent waves). Therefore most island nations may need to plan for multiple additional interventions (e.g., screening and quarantine) to raise the probability of remaining pandemic free or achieving substantial delay in pandemic arrival.