In spring of 2020, COVID-19 rapidly escalated from a localized outbreak into a global pandemic. The United States and many other countries attempted to slow the virus by closing their borders to foreign travelers, particularly those arriving from countries with high infection rates.

But were infected foreigners truly the largest source of new infections in the U.S. or elsewhere? A new study combining mathematical models with data on early-pandemic travel records and infection patterns suggests otherwise. 

“We found that visitors from abroad are less likely to bring new infections into remote countries than travelers who are returning home with new infections,” said Michael Landis, an associate professor of biology and the senior author of the study, published in the prestigious journal PNAS. The lead author was postdoctoral researcher Albert Soewongsono. 

In other words, Landis said, the coronavirus infections they studied were more often “pulled” into countries by returning residents, who were infected during their trip, than “pushed” across borders by international visitors.

Landis, Soewongsono, and co-author Ammon Thompson from the Oak Ridge Institute for Science and Education developed a mathematical model describing how viruses spread between countries. The model accounted for three possible routes of international transmission: from visitors to residents, from residents to visitors, and among visitors. It also accounted for the evolutionary relationships of the transmitted viruses. 

“We tried to come up with a more realistic representation for how people move while they’re on trips, and how that impacts disease transmission,” Landis said. 

The team also examined travel data and infection data involving several European countries — including France, Italy, and Germany — and the Hubei province of China, the presumed epicenter of the outbreak. “It's really hard to get private information about individuals, including their infection status and travel history,” Landis said. “But we were able to look at country-level travel and tourist information to quantify how people were moving at the earliest stages of the pandemic.” 

The findings highlight a risk familiar to many travelers: It is easy to contract a virus during a business trip or vacation. “Your behavior changes when you’re traveling,” Landis said. Travelers can then unknowingly carry the virus home, potentially sparking additional infections. “Maybe people were traveling recklessly, and they weren't aware that they could bring it back with them.”

International travelers can also introduce viruses to the places they visit, Landis said. However, the mathematical model and real-world data suggest such events were relatively rare. That may be partly because international visitors were presumably going about their normal lives before traveling rather than engaging in the higher-contact behaviors associated with tourism.

Other factors may also make international visitors less likely than returning residents to introduce viruses, Landis said. “Many people will cancel a trip to a foreign country if they start feeling sick,” he said. By contrast, people already overseas may be highly motivated to return home even if they feel ill. 

Although the study focused on a single virus and a limited number of countries, the findings could help inform responses to future outbreaks around the world, Landis said. “There was a lot of xenophobia at the beginning of COVID,” he said. “People thought that foreigners were bringing disease, but our study suggests that fear was misplaced.”

Countries would have to completely close their borders to foreign visitors and returning residents alike to prevent travel-related spread during future outbreaks — an unlikely scenario. On an individual level, travelers can take precautions beyond basic measures such as washing their hands regularly and wearing masks in crowded settings. 

“When you’re traveling abroad, it makes sense to be selective about who you interact with,” Landis said. “Then again, meeting strangers and reuniting with friends and family is part of the joy of the experience. It’s a tough balance.”

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