Countries restrict the overall extent of international travel and migration to balance the expected costs and benefits of mobility. Given the ever-present threat of new, future pandemics, how should permanent restrictions on mobility respond? A simple theoretical framework predicts that reduced exposure to pre-pandemic international mobility causes slightly slower arrival of the pathogen. A standard epidemiological model predicts no decrease in the harm of the pathogen if travel ceases thereafter and only a slight decrease in the harm (for plausible parameters) if travel does not cease. We test these predictions across four global pandemics in three different centuries: the influenza pandemics that began in 1889, 1918, 1957, and 2009. We find that in all cases, even a draconian 50 percent reduction in pre-pandemic international mobility is associated with 1-2 weeks later arrival and no detectable reduction in final mortality. The case for permanent limits on international mobility to reduce the harm of future pandemics is weak.
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