The COVID-19 pandemic began at different times in different regions, which puts various countries at different levels of risk for new cases. These cases could come from outside (imported cases) or as a result of local contacts or even community spread.
Now, a new study published in July 2020 on the preprint server medRxiv* shows that the latter is likely to cause 90% or more of local epidemics and that stringent bans on international travelers are therefore unlikely to affect the course of the local outbreak significantly.
Various countries all over the world have put travel restrictions in place, closing off national and international boundaries. These measures also include mandatory quarantine or self-isolation for travelers from outside the country. The usefulness of these measures can be assessed only by combining the travel data by country with the estimated prevalence and incidence, which will allow a prediction of the risk of importing the disease as compared to the risk of local spread within a country.
The Cost of International Travel Bans
Travel restrictions have a high cost as they affect global mobility required for trade, tourism, education, and labor force recruitment. Already, COVID-19 is estimated to have already cost up to about 3% of the world’s gross domestic product (GDP), according to the United Nations Conference on Trade and Development (UNCTAD), in the area of tourism alone.
Socially, too, restricting travel prevents family and friend interactions, opportunities for international education, and career advancement steps. Thus, the challenge is to keep the area of travel restriction as reasonably free as possible, unless the cost is so high as to surpass any reasonable chance of net benefit. This will also help conform to the International Health Regulations (2005) guidelines that these bans “shall not be more restrictive of international traffic and not more invasive or intrusive to persons than reasonably available alternatives that would achieve the appropriate level of health protection.”
The Limitations of International Bans
When the destination is reasonably free of the disease and the country of origin has a high prevalence, travel restrictions are helpful, as when Wuhan, and China in general, placed bans on travel outside its geographical boundaries. At a point when local outbreaks are large and can sustain local transmission, such measures become less effective.
This is seen with the restrictions placed on travelers from Europe to the U.S. in the second week of March 2020. Since the U.S. had already developed numerous foci of infection, mostly from European travelers, by then, a travel ban was likely to be useless in preventing the spread of the virus.
Percentage reduction in passenger numbers required for countries in scenario B where imported cases account for more than 1% of local incidence to bring that proportion below 1%. Countries are grouped by United Nations Region (Oc. is Oceania)
The Study: Assessing Traveler Contribution to Local Outbreak
The current study used these combined measures, including the number of airline travelers and the number of daily flights, the adjusted prevalence, and incidence of disease, in 142 countries. The researchers then found the risk of importation concerning local transmission. The aim was to provide a tool to help guide policies on travel restriction – helping to apply them where they are useful and avoiding them where they make little impact.
Whereas travel restrictions first applied to countries that had high early numbers of cases, notably China, Iran, and Italy, their scope was expanded later to include many more countries of origin. In fact, according to the World Health Organization (WHO), every country had some form of travel ban by April 20, 2020 – which amounts to “the most extensive travel restrictions in history,” the researchers say.
No Travel Restrictions, No Reduction in Travel Volume
When there is no expected change in travel patterns in 2020 relative to 2019, without travel restrictions, the researchers predict that imported cases would contribute to less than a tenth of local cases, in 103 of 142 countries. In 48 countries, imported cases would account for less than 1% of total cases.
Using data from the OpenSky dataset, the researchers revised the expected number of international travelers downwards by about 70% in 2010 compared to 2019. They also considered the situation in which flight occupancies decline to 75% and 50% of the 2019 situation.
When a reduction in international travel is factored in because of self-imposed isolation or preventive quarantine, in the absence of travel restrictions, they find little change in the risk of case importation. The imported cases will account for less than 10% of the total in 109-123 countries, and less than 1% in 61-88 countries. The upper and lower limits on the number of countries are due to the two different assumptions made about the volume of reduction in travel, at 25% and 50% of 2019 levels, respectively.
This is best seen with countries that have 10% or less of new cases originating with imported cases, such as China and Thailand, as well as Australia. In Europe and Latin America, the contribution of imported cases is below 1% of the local incidence.
In this scenario, several countries that have more than 1% of their local incidence caused by international imports would need to block the majority of their international visitors to reduce the imported cases to below 1% of local cases. A few need to reduce the number of arrivals by only 25% or so, blocking only travelers from countries with the highest prevalence.
Many countries like South Korea, Australia, and Italy passed the first wave epidemic peak, and some have very low current levels of new cases. In most of the countries studied, the number of imported cases would be so small as to make little difference in the overall local outbreak.
Some countries, however, would require travel restrictions. These include those which have moderate outbreaks, as in the American continents, and also typically send and receive a large number of travelers to and from countries with high COVID-19 prevalence. If there are no travel restrictions in place, this could lead to the imported cases strengthening the moderate local outbreak.
A third case involves those countries which have brought their local epidemics under control. In such cases, as with New Zealand, the predicted number of importations would be higher than the local incidence, and could thus trigger a second wave local epidemic. Here, almost all international travel would have to be banned.
Those countries which most need international travel bans are “those that have both good international travel connections and very low local COVID-19 incidence.”
Despite the apparent limitations of the study, such as overlooking road and rail traffic, and not considering the impact of international departures, as well as not factoring in the likelihood that infected people are less likely to travel for many reasons than uninfected people, the study shows most countries do not require strict travel restrictions.
The researchers conclude, “Our results suggest that in May 2020, travel restrictions may have done little in most countries to change the course of local epidemics, and may not be justified given the high economic and social costs required to prevent the arrival of travelers representing less than 10% (or in many cases, less than 1%) of new cases. “ They hope that policymakers will be equipped to make better decisions about which restrictions to put in place based on the methods they present.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
Russell, T. W. et al. (2020). The Effect of International Travel Restrictions on Internal Spread Of COVID-19. medRxiv preprint. doi: https://doi.org/10.1101/2020.07.12.20152298. https://www.medrxiv.org/content/10.1101/2020.07.12.20152298v1