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Background In the context of an outbreak the natural boundaries of islands can allow for control of movements between populations. We estimate the risk of introduction of COVID-19 to each of the Hebridean islands situated off the west coast of mainland Scotland due to individual movements, and explore control strategies to mitigate this risk.Methods We use a combination of real human mobility data and census data to generate seasonally varying patterns of human movements amongst the Hebrides and from elsewhere. We consider three distinct periods: each of summer and winter 2019, illustrating a year prior to the pandemic, and summer 2020 illustrating a "pandemic summer". Movements during these periods serve as input to simulate COVID-19 transmission from the mainland to the archipelago in a stochastic meta-population model allowing us to explore the impact of seasonal variations on the risk of introduction and the effectiveness of non-pharmaceutical interventions.Results Despite strong seasonality in movement patterns, partly driven by tourism, for islands closely connected to the mainland there is evidence of substantial risk of disease introduction even over winter. In summer, when the risk is the highest, some islands can delay the introduction of COVID-19 by over six weeks, i.e. beyond the summer holiday period, through a 70% reduction of movements. Conclusion A high introduction risk in winter will be of particular concern if COVID-19 becomes a seasonal respiratory infection affecting temperate areas in winter concomitantly with other seasonal infections such as flu.For some islands, control of movements in peak summer tourist season has the potential for delaying the introduction risk beyond the summer holiday period, i.e. beyond a period of high mobility of people, potentially inducing a risk for rapid spread. Such measures would be particularly relevant in the occurrence of a variant escaping the vaccine given the current progress of the vaccine roll-out. However, such restrictions must be balanced against indirect negative economic impacts that might result. |
