To estimate the risk of imported COVID-19 cases relative to local transmission, researchers combined data on airline passengers and flight frequencies with estimates of COVID-19 prevalence and incidence (adjusted for underreporting and asymptomatic cases). The number of imported cases was calculated by multiplying the prevalence of COVID-19 on a specific date by the number of travelers to a country on that same date.

Northwestern University

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This paper focuses on the effects that instigating national/international travel restrictions had on controlling local COVID-19 pandemics in May 2020. The authors came to the conclusion that overall, imported cases don't contribute much to local transmission, unless there are very strong travel links to countries with high COVID-19 cases or the country from which people are travelllng has controlled their outbreak.

- prior evidence
- methods
- results
- implications

The effect of international travel restrictions on internal spread of COVID-19

Russell T., Wu J., et al. 14 July 2020. The effect of international travel restrictions on internal spread of COVID-19. BioRxiv. https://doi.org/10.1101/2020.07.12.20152298.

Different countries implement different travel restriction strategies according to the severity of their COVID-19 pandemic. "Policies include border closures, flight suspensions, quarantine and self-isolation on international arrivals". However, no study has combined "country-specific travel data, prevalence estimates and incidence estimates to assess the global risk of importation relative to current local transmission within countries". So far it has only been clear that travel restrictions benefit countries with zero/few cases in the destination country, but once case counts reach larger values travel restrictions don't help much. This paper "provides information to countries about the potential benefit of international travel restrictions by comparing the number of cases resulting from international travel to those resulting from local transmission in 142 countries. "

Prior Evidence

The study came to the conclusion that in May 2020 travel restrictions have done little in most countries to control local epidemics. This leads to the conclusion that severe restrictions are not justified when taking into consideration the extreme economical and social costs that result from these restrictions. Only a few countries with very few COVID-19 cases, such as New Zealand, would directly benefit from travel restrictions since imported case amounts might exceed local levels.

Implications for Countries with COVID-19-Related International Travel Bans

Methods: Estimating the Risk of Imported COVID-19 Cases

A May 2020 study on the effect of international travel restrictions on the spread of COVID-19 found that even when no travel restrictions were implemented, imported cases accounted for less than $$ 10 \% $$ of local cases in $$ 109 $$ out of $$ 142 $$ countries. Furthermore, to reduce imported cases to below $$ 1 \% $$ of local incidence, most countries would need to ban the majority of their international travel, which is often unfeasible.

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