Travel Restrictions and the Spread of COVID-19 – What Does the Research Say?
Many countries have enacted travel restrictions in response to the spread of COVID-19 that originated in China. For instance, President Trump announced a partial travel ban on China in late January that went into effect on February 2, 2020. Trump’s partial travel ban exempted American citizens, lawful permanent residents, and many others. Those who returned were supposed to be medically screened and quarantined if they showed symptoms. Those quarantines and medical screening were likely inadequate as we saw after the government imposed travel restrictions on Europe and returning Americans overwhelmed the airports. At least 36 other countries also imposed travel restrictions on people coming from China at about the same time.
The goal of the travel restrictions was to slow the spread of COVID-19. Clearly, COVID-19 is just about everywhere, but the important question is whether the travel bans slowed the spread enough to either somewhat flatten the curve or shift the curve to the right to give enough time for health care providers, civil society, and governments to prepare. It makes intuitive sense that travel bans could have slowed the spread of COVID-19 because it originated in one part of China and spread predictably via airline routes. A recent Rasmussen poll shows that the travel bans on China and Europe are popular if they limit the spread of COVID-19.
The government has the power to limit the spread of diseases through travel restrictions. The appropriateness of a travel restriction depends upon how effective it is at reducing the spread of disease and reducing its associated costs in health, lives, and to the economy relative to the costs incurred by such a restriction. There is a growing body of academic research on travel restrictions imposed by governments in response to the COVID-19 pandemic. Although it is early and there are problems with the COVID-19 data available, travel restrictions likely delay the spread of COVID-19 in modeled simulations by up to a few weeks.
Academic Research – Epidemiological Models
Epidemiological research on how travel restrictions impede the spread of COVID-19 is almost entirely theoretical and model-based. These papers usually begin by building or adapting a model of COVID-19 transmission via transportation routes. Travel restrictions reduce or remove nodes from transportation networks, limiting the spread of the virus in destination countries where those restricted-travel nodes reside. However, these travel restrictions don’t reduce travel with all other nodes in the international transportation network. Thus, if COVID-19 is traveling between other nodes in other non-restricted countries, travel restrictions on highly-infected countries like China merely delay the spread. In other words, travel restrictions aren’t as ineffective as closing the barn door after the horse escapes, but they aren’t that much better.
A recent article in the journal Science models how China travel restrictions affected the global spread of COVID-19. They use an “individual-based, stochastic, and spatial epidemic model” called Global Epidemic and Mobility Model (GLEAM) that integrates real-world data on travel between travel nodes (cities and population centers) and the transmissibility of COVID-19.
Individuals inside of GLEAM go through several different phases with respect to COVID-19: They can be susceptible to the disease, have latent or inactive infections, be currently infectious, or are removed due to recovery or death. Those who are susceptible can acquire COVID-19 through contact with infectious individuals. Those susceptible individuals who become infected will go through a latent period and proceed to become themselves infectious. Those people who are infectious will then move toward the removed stage where they are no longer able to infect others because they were isolated, hospitalized, died, or fully recovered and acquired immunological memory. Importantly, the authors also consider how changes in the transmissibility of COVID-19 affect the spread of the disease relative to and in conjunction with travel restrictions. The authors of the paper estimated the time that an individual spends in each phase based on COVID-19 data available before the paper was published on March 6, 2020.
Next, the authors run the GLEAM model with different reductions in international travel. It’s difficult to disentangle the effect of travel restrictions imposed by different countries because of many imposed bans on travel from China at different times. Furthermore, China imposed internal travel restrictions in response to COVID-19. Thus, the authors simplify their model by considering the effects of a 40 percent and 90 percent reduction in travel to and from mainland China, both beginning on February 1, 2020. Separately, they also model how a travel ban on the city of Wuhan, which began on January 23, specifically affected the spread of the disease in the rest of Mainland China and internationally.
The Wuhan-only travel restriction reduced the international transmission of COVID-19 cases outside of Mainland China by 77 percent by early February. However, the numbers quickly rebounded. Within 2-3 weeks of the Wuhan travel restriction, the number of international cases of COVID-19 that spread to countries via airline transportation resumed its pre-Wuhan-restrictions numbers after 2-3 weeks. In other words, the Wuhan travel restriction likely reduced the spread of COVID-19 in other countries by a few weeks.
As for a February 1, 2020 travel restriction on the rest of Mainland China, the authors estimate that a 90 percent reduction in international travel would only modestly affect the pandemic’s international trajectory such that the number of cases crossing borders per day will return to the pre-Mainland China travel ban level by February 16th. A 40 percent reduction in international travel from Mainland China beginning on February 1, 2020 would have an even smaller effect. In that scenario, the number of cases crossing borders per day returns to the pre-travel ban level on February 3, 2020.
The most important finding of the Science article is that reducing transmissibility does more to slow the spread of COVID-19 than travel restrictions. A 90 percent reduction in travel merely reduces one node in a travel network whereby COVID-19 spread. By lowering transmissibility across the world, even if only by 25 percent, the spread of COVID-19 through every node in that network is reduced greatly.
I contacted some of the authors of the Science article to ask for their code so I could replicate their model with different travel restriction scenarios, holding transmissibility constant. Additionally, I asked them to rerun their model under different assumptions and by looking specifically at the spread of COVID-19 inside of the United States. Unsurprisingly, they are extremely busy and haven’t responded yet. The biggest issue with this paper is that it’s just a model, a very useful one that increases our understanding of how COVID-19 spread, but it has not been empirically tested.
Another paper published in the Proceedings of the National Academy of Sciences looks at how Chinese-government imposed travel restrictions impeded the exportation of COVID-19 cases abroad. It found that by January 12, 2020, the daily risk of exporting a single case of COVID-19 to another country exceeded 95 percent and the likely first export occurred in December 2019. The Chinese government didn’t impose travel restrictions out of Wuhan until January 23, 2020. The authors estimate that the Chinese travel lockdown reduced the number of COVID-19 importations in other countries by about 80 percent by mid-February. They wrote:
travel restrictions cannot be expected to fully arrest the global expansion of COVID-19, but may decrease the rate of case exportations if enacted during the early stages of the epidemic . . . At this early stage of the COVID-19 epidemic, this decrease in exportation rates from mainland China could delay the onset of outbreaks in cities yet to be affected. This obstruction of importation events will be critical to preparing an effective public health response for when a local COVID-19 outbreak arises.
A third paper published in the Journal of Clinical Medicine ran a model about how the reduction in travel with China affected the number of COVID-19 cases in Japan, whether it delayed a “major epidemic” there, and how long the epidemic was delayed. It found that those restrictions reduced the probability of a major epidemic in Japan from 7 percent to 20 percent, which resulted in a median time delay of a major epidemic of two days. That effect was produced by reducing the number of COVID-19 cases arriving in Japan by 70.4 percent from January 28-February 7, 2020.
A fourth non-peer reviewed paper modeled the effect of Australia’s restriction on Chinese travel that was imposed on February 1, 2020. This paper is the first to look at importation bans specifically rather than reductions in overall travel. The authors found that Australia’s travel ban reduced the number of cases and deaths from COVID-19 by about 87 percent over a 400-day time period projected into the future. That is a huge effect in the model, but the paper does not consider the spread of COVID-19 to Australia from countries other than China. Including the spread of COVID-19 to Australia from other countries would certainly reduce the effectiveness of the Australian ban.
The existing literature finds that travel restrictions likely delayed the spread of COVID-19 internationally by a short period of time, from a few days to several weeks. The next question is whether these short delays made a difference.
Did the Delaying the Spread of COVID-19 Matter?
Many of the papers above model how travel restrictions delayed the spread of COVID-19, often by a short period of time. There are many reasons why travel restrictions didn’t do more to reduce the spread of COVID-19 in these models. First, many of the travel restrictions on China excluded many people. Viruses don’t discriminate by nationality, so American citizens could have brought the virus back to the United States or other countries after the travel ban. Second, the travel restriction was imposed too late. The virus had escaped China already and it was being transmitted to healthy people around the world. The travel restrictions also didn’t block travelers from other countries.
Third, the travel ban created a false sense of security that delayed other more effective methods of containing COVID-19, such as instituting social distancing sooner. In other words, the travel ban was always an ineffective policy that substituted for more effective ones like those adopted in Singapore. Other methods of reducing COVID-19 transmission are effective. To the extent that travel restrictions substituted for those policies, as they likely did in the United States, it likely undid any benefits by delaying other actions.
A delay in the spread of COVID-19 could purchase precious time to flatten the curve so that seriously ill patients won’t overwhelm hospitals, intensive care units, and die in larger numbers as a result. But that only works if the government, civil society, and business use the extra time to prepare for the coming pandemic. Like every other disaster, few were going to act until it reached certain crisis proportions. There was evidence that COVID-19 was building as a looming threat for months – in mid-February, I stocked up on emergency supplies, extra cleaning supplies, hygienic equipment, and ammunition because I was worried about COVID-19. I wasn’t the only one nor was I particularly prescient.
Conclusion
Travel restrictions, including limits on emigration and immigration, delayed the spread of COVID-19 by a few days to a few weeks. The above research papers are limited as they just model and project COVID-19 cases rather than empirically test how restrictions affected the actual spread of the disease, but they are valuable starting points in evaluating this crucial policy tool. Travel restrictions could have delayed the onset of a crisis mentality by a few weeks and shifted the curve to the right rather than flattening it. Shifting the curve to the right, in addition to preparing for a pandemic by adopting other policies to reduce transmission, could have been an effective strategy. But by themselves, travel restrictions do little but delay the onset of a crisis mentality and shift the curve to the right rather than flattening it.
