The influence of cross-border mobility on the COVID-19 epidemic in Nordic countries

Restrictions of cross-border mobility are typically used to prevent an emerging disease from entering a country in order to slow down its spread. However, such interventions can come with a significant societal cost and should thus be based on careful analysis and quantitative understanding on their effects. To this end, we model the influence of cross-border mobility on the spread of COVID-19 during 2020 in the neighbouring Nordic countries of Denmark, Finland, Norway and Sweden. We investigate the immediate impact of cross-border travel on disease spread and employ counterfactual scenarios to explore the cumulative effects of introducing additional infected individuals into a population during the ongoing epidemic. Our results indicate that border restrictions can significantly influence the course of an epidemic, but this impact is highly contingent on the prevailing epidemic status of the involved countries. In particular, there are several instances in which the contribution of cross-border movement was found to be negligible. Our findings underscore the critical importance of accurate data and models on both epidemic progression and travel patterns in informing decisions related to inter-country mobility restrictions. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work has been supported in part by the project 105572 NordicMathCovid as part of the Nordic Programme on Health and Welfare funded by NordForsk. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study used 1) Publicly available data on number the number of Covid-19 hospitalizations. The data was aggregated to the number of cases per week per country 2) Publicly available data on cross-border travel 3) Private data on the airplane cross-country travel. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that no patient/participant/sample identifiers were not known to anyone in the research group. All used data is published alongside the paper, at https://gitlab.com/2pi360/covid\_model\_mobility_public I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced are available online at [https://gitlab.com/2pi360/covid\_model\_mobility_public][1] [1]: https://gitlab.com/2pi360/covid_model_mobility_public