Abstract
BackgroundCountries have restricted international arrivals to delay the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These measures carry a high economic and social cost, and might have little effect on COVID-19 epidemics if there are many more cases resulting from local transmission compared with imported cases. Our study aims to investigate the extent to which imported cases contribute to local transmission under different epidemic conditions.MethodsTo inform decisions about international travel restrictions, we calculated the ratio of expected COVID-19 cases from international travel (assuming no travel restrictions) to expected cases arising from internal spread, expressed as a proportion, on an average day in May and September, 2020, in each country. COVID-19 prevalence and incidence were estimated using a modelling framework that adjusts reported cases for under-ascertainment and asymptomatic infections. We considered different travel scenarios for May and September, 2020: an upper bound with estimated travel volumes at the same levels as May and September, 2019, and a lower bound with estimated travel volumes adjusted downwards according to expected reductions in May and September, 2020. Results were interpreted in the context of local epidemic growth rates.FindingsIn May, 2020, imported cases are likely to have accounted for a high proportion of total incidence in many countries, contributing more than 10% of total incidence in 102 (95% credible interval 63–129) of 136 countries when assuming no reduction in travel volumes (ie, with 2019 travel volumes) and in 74 countries (33–114) when assuming estimated 2020 travel volumes. Imported cases in September, 2020, would have accounted for no more than 10% of total incidence in 106 (50–140) of 162 countries and less than 1% in 21 countries (4–71) when assuming no reductions in travel volumes. With estimated 2020 travel volumes, imported cases in September, 2020, accounted for no more than 10% of total incidence in 125 countries (65–162) and less than 1% in 44 countries (8–97). Of these 44 countries, 22 (2–61) had epidemic growth rates far from the tipping point of exponential growth, making them the least likely to benefit from travel restrictions.InterpretationCountries can expect travellers infected with SARS-CoV-2 to arrive in the absence of travel restrictions. Although such restrictions probably contribute to epidemic control in many countries, in others, imported cases are likely to contribute little to local COVID-19 epidemics. Stringent travel restrictions might have little impact on epidemic dynamics except in countries with low COVID-19 incidence and large numbers of arrivals from other countries, or where epidemics are close to tipping points for exponential growth. Countries should consider local COVID-19 incidence, local epidemic growth, and travel volumes before implementing such restrictions.FundingWellcome Trust, UK Foreign, Commonwealth & Development Office, European Commission, National Institute for Health Research, Medical Research Council, and Bill & Melinda Gates Foundation.
Highlights
COVID-19 is an illness caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which was first detected in Wuhan, China, in late 2019
Recomm endations about international travel restrictions cannot be applied uni formly, but instead need to take into account country circumstances. In this Article, we provide information about the potential benefit of international travel restrictions when countries are experiencing different epidemic conditions, by comparing the number of cases resulting from inter national travel to those resulting from local transmission in 162 countries
Total incidence was inferred from deaths occurring in a country, so would include cases arising from local transmission as well as imported cases, since they would presumably contribute to COVID-19 deaths in that country
Summary
COVID-19 is an illness caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which was first detected in Wuhan, China, in late 2019. It has been spread by travellers to almost every country in the world, and was declared a pandemic by WHO on March 22, 2020.1 In the absence of effective pharma ceutical measures for prevention and treatment, govern ments have imposed a range of response measures to delay the spread of SARS-CoV-2, and enable health systems to cope with the expected sharp rise in healthcare demand. One such intervention that has been widely used is international travel restrictions. The World Tourism Organi zation reports that every country in the world had imposed some form of COVID-19-related travel restriction by April 20, 2020, marking the most extensive travel res trictions in history. the restrictions implemented differ from country to country and include border closures, flight suspensions, and quarantine and self-isolation for www.thelancet.com/public-health Vol 6 January 2021
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