The biggest factor in the death rate from the first wave of coronavirus cases was international travel, according to a new study from researchers at the University of Aberdeen.

Researchers assessed death rates in the 37 hardest-hit countries, looking at factors including urban population, population density and arrivals at the border. Countries analyzed included the U.S., the U.K., Spain, France, Italy and Brazil.

According to the research, published in BMJ Open, each new one million international arrivals was associated with a 3.4 percent spike in the mean daily increase of deaths from the virus.

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“Our assessment of available data indicates that very early restrictions on international travel might have made a difference in the spread of pandemic in western Europe, including the UK,” co-author Tiberiu Pana said in a statement. “These findings are particularly important as the world looks to control future waves and strains of the COVID-19 pandemic and prevent related deaths.”

The data also indicate a country’s rates pf Bacillus Calmette–Guérin vaccination may also be associated with a reduction in death rates, Tiberiu said.

“Nevertheless, these associations were weaker and further work looking at individual patients is required to clarify these potential relationships,” he added.

The U.S. imposed restrictions on certain foreign nationals from China and some European nations early in the pandemic. President Biden last week signed a directive maintaining travel restrictions on most of Europe and Brazil and added South Africa to the list.

The first cases of new variations believed to have originated in South Africa and England have been identified in the U.S.

On the other side of the Atlantic, Scottish First Minister Nicola Sturgeon has announced stricter travel restrictions on arrivals from nations that have detected the South African strain. Sturgeon said that current plans by the British government to sequester high-risk arrivals in hotels was insufficient. Scottish officials will introduce a “much more comprehensive” approach to “managed quarantine” for such arrivals, she said, according to the BBC.