Are travel restrictions helpful to control the global COVID-19 outbreak?

Dear Editor,

Before mid-March 2020, some countries adopted limited air traffic restrictions to prevent the spread of the COVID-19 outbreak. After the World Health Organization (WHO) declared COVID-19 a global pandemic on 11 March 2020, more and more countries started to take more stringent actions in regard to entry restrictions for travelers arriving from other countries by air [1]. This correspondence presents findings on the probable effects of travel restrictions on the COVID-19 outbreak across different countries.

Some studies have investigated an association between flight connections in a particular country and the spread of COVID-19 [2,3], but very few research has examined the effects of travel restrictions on the COVID-19 pandemic across different countries. We therefore fill this research gap by investigating the relationships among the 10 countries with the most reported cases of COVID-19 [4], their daily number of reported flights [5], and the connectedness of these countries from February to November 2020, as shown in Fig. 1 .

Fig. 1

Time series plots of the daily reported flights and confirmed COVID−19 cases in 10 countries and network density.

We calculate the connectedness of countries using network density, referring to the ratio of the number of connections with respect to the maximum possible number of connections among countries [6]. Specifically, we define , where is the number of confirmed cases in country i on day t. We then define a connection in the pandemic network whenever the correlation between and is > 0.5. We also count , the number of connections in the pandemic network, to determine the network density on day t. This network density measures the connectedness of countries by the proportion of pairs of countries that are connected due to ‘co-movement’ in terms of confirmed cases. The higher the network density, the higher the likeliness that the countries' number of confirmed COVID-19 cases will increase simultaneously.

As illustrated in Fig. 1, there were relatively few flight restrictions among the 10 countries in February 2020. Network density, however, started to spike in mid-February and reached a peak in late February, signifying the onset of the COVID-19 pandemic, even though the numbers of daily reported confirmed cases in the 10 countries were still low. The number of cases began to climb from mid-March 2020, around the second peak in the network density. Also significant is the way in which the decrease in the network density started in mid-March, which coincided with the reduction of daily reported flights. We posit that the ‘co-movement’ in the daily reported flights and network density time series may be used as evidence that air travel restrictions could have effectively reduced human mobility, which was useful in preventing the further spread of COVID-19 internationally.

It is generally recognized that countrywide air travel is a main facilitator of the global transmission of COVID-19 [3]. Some countries imposed relatively strict restrictions on air travel by reducing more than 90% of their daily flights when the WHO declared the COVID-19 outbreak a pandemic on 11 March 2020. Their travel restriction policies seem to have helped reduce the spread of COVID-19 worldwide, as we can see that the number of daily reported confirmed cases in these countries is relatively stable. However, starting from around July, some countries reopened their borders or resumed some regular travel. Correspondingly, there is an increasing trend in the network density and the daily reported confirmed cases began to fluctuate more and become less controllable.

To conclude, we can see some evidence of the probable effect of travel restrictions on the spread of the COVID-19 pandemic, especially in the early stages of the virus's outbreak. It would be useful to re-examine the travel policies of these countries and, if necessary, adopt more stringent travel reductions. However, other non-pharmaceutical interventions, such as quarantine and isolation, social distancing and hygiene measures, are also crucial to preventing the spread of the virus due to local and community transmission, where air travel restrictions may have less effect.

Author contributions

AMYC and MKPS designed the research. MKPS and JNLC collected and analyzed the data. AMYC, AT, and MKPS interpreted the results. AMYC drafted the manuscript. AT and MKPS finalized the manuscript. All authors read and approved the final version of the manuscript.

Source of funding

This work was partially supported by the Hong Kong RGC General Research Fund (grant number 16307217). The funding body had no role in study design, data collection and analysis, preparation of the manuscript, or the decision to publish.

Declaration of competing interest

All authors have no conflicts of interest.