Probability to escape a major epidemic
300 400 500 600 700 Number of travelers during pandemic period
Note: For the basic reproduction number R0 being 1.5 (solid line), 2.25 (dashed line) or 3.0 (dotted line), respectively.
Figuireili1n aoPICcTpdeuringathe pandemicbperiodber of visitors Probability to escape a major epidemic by number of visitors arriving on a PICT during the pandemic period.
quarantine  or home quarantine with intensive mon- itoring; possibly the routine provision of antivirals to incoming travellers; pre-pandemic vaccination of their populations (if an appropriate vaccine became available); enhanced capacity for disease surveillance in the commu- nity and for rapid outbreak control capacity. As nearly 75% of infected travellers arrive without symptoms, entry screening based on the travellers' symptom states alone only slightly improves the escape probability (e.g. it increases Tonga's escape probability from 32 to 46% for the R0 = 1.5 scenario with 99% travel reduction) if all symptomatic travellers are prevented from infecting any- body.
Our calculations assume that travel reduction remains constant during the whole period of the global pandemic. Higher numbers of travellers may temporarily be admit- ted from regions which are not or only slightly afflicted by the pandemic, but this strategy may be too difficult to implement because it would require the travel history of each arriving traveler to be verified. An alternative to these interventions is planning for complete border closure (i.e., practically 100% travel volume reduction) at the first sign of a global pandemic - a response that some PICTs used successfully during the 1918/19 influenza pandemic .
Even rigorous travel volume reductions might, however, be difficult for those PICTs that partially depend on food imports and other critical imports (e.g., medical supplies). Nevertheless, some PICTs might be able to facilitate ongo- ing trade by aircraft and shipping while keeping the crews of these vessels entirely separated from the local popula- tion (e.g., with high security unloading facilities where the
crew never actually disembark while their vessel is unloaded). Others could enhance food self-sufficiency by increasing fishing and diverting export crops (e.g., coco- nut oil) for use as food.
Pandemic severity varies greatly with the experience of the current swine-origin (H1N1) influenza pandemic (at least to mid-2009) indicating a severity that might even be less overall than seasonal influenza. Therefore, good data on severity at the start of a pandemic (or a new pandemic wave) will help island nations decide if mandated travel volume reduction is a worthwhile intervention. Key varia- bles for such early decisions from affected countries (espe- cially developing countries) are hospitalisation rates and case fatality ratios.
Also of note that some actions that would assist with severe travel volume reductions during pandemic influ- enza might be worthwhile in their own right. One exam- ple is building infrastructure to improve access to the Internet and to allow videoconferencing. Diversifying island economies (to reduce reliance on tourism) may also cushion island economies against other natural disas- ters and routine fluctuations in tourism numbers.
Although travel restrictions may not be sufficient to pre- vent the successful importation of an infection, they should (at least on average) delay it. Scalia Tomba and Wallinga  demonstrated with a simple mathematical model that an overall travel reduction by 99% should delay an epidemic on an island by about three weeks if R0 is approximately 2.
Limitations This analysis made many simplifying assumptions. It could potentially be improved by developing a more com- plex stochastic model that used log-normal or gamma-dis- tributed sojourn times (rather than the exponential distributions used here). Such a model would also be able to provide information on the average time of pandemic arrival. Improved modelling (including combining addi- tional border control interventions with travel reduc- tions) may not only facilitate pandemic planning among PICTs but also help other island nations (and sub- national island jurisdictions) in the Caribbean, Southeast Asia and off the coast of most continents.
Although we considered a range of values of R0, it is con- ceivable that in a global pandemic the effective R0 would decline after the first few months of pandemic emergence. This decline is because many countries around the world are very likely to adopt social distancing and other control measures. Possibly after some months, relevant technolo- gies such as a pandemic strain vaccine might also become available (and start to be used by those planning to
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