Researchers advised governments to consider local coronavirus 2019 (COVID-19) incidence rates and epidemic growth before implementing travel restrictions. These findings were published in Lancet Public Health.

Prevalence and incidence rates for COVID-19 in May and September 2020 were combined with detailed flight data from May and September 2019 and 2020 to formulate risk ratings for each country. Local and transported infections were estimated, and the rate of spread was projected to give guidance on implementing travel restrictions.

Data from May were available for 136 countries and data from September for 162 countries. The risk ratio was based on the imported cases to total incidence for different passenger reduction scenarios.

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In May 2020, local epidemics were most affected in regions where COVID-19 importation exceeded 10% (102; 95% credible intervals [CrI], 63-129 countries), which occurred in countries that had no change to travel volume between May of 2019 and 2020. Based on the travel volumes reported by OpenSky, the 10% importation rate was more likely to have occurred in 74 (95% CrI, 33-114) countries and up to 10% in 106 (95% CrI, 50-140) countries.

In September 2020, assuming similar travel volumes as in 2019, the 10% threshold would be exceeded in 56 (95% CrI, 22-112) countries and up to 10% in 106 (95% CrI, 50-140) countries. With the observed reduction of travel, the 10% threshold was likely exceeded by 37 (95% CrI, 8-85) countries and up to 10% in 125 (95% CrI, 65-162) countries.

Among the 44 countries with risk ratings below 1% in September 2020, 22 of these countries did not have high risk R estimates. It is likely that these countries would have posed little risk for COVID-19 outbreaks if travel restrictions had been lifted.

The investigators estimated the proportion of international arrivals that needed to be averted in order to reduce the rate of imported cases to less than 1% for each of the 118 countries that likely had high rates of imported COVID-19 cases. The diversion rates ranged from high (~100%: Cyprus, Thailand, Vietnam, China, and Mauritius) to low (<5%: North Macedonia, Kazakhstan, and Congo).

These data may have been limited by the fact that only 1 mode of transportation (flight) was incorporated into this analysis.

These data indicated that strict travel restrictions may be unjustified in some countries (n=44), especially among countries with very low local COVID-19 epidemics. Therefore, governments should make detailed decisions about travel restrictions during the COVID-19 era.


Russell TW, Wu JT, Clifford S, Edmunds WJ, Kucharski AJ, Jit M; Centre for the Mathematical Modelling of Infectious Diseases COVID-19 working group. Effect of internationally imported cases on internal spread of COVID-19: a mathematical modelling study. Lancet Public Health. 2020;S2468-2667(20)30263-2. doi:10.1016/S2468-2667(20)30263-2.