In early 2020 the U.S. government reacted to worrisome death figures for COVID-19, the SARS-CoV-2 form of coronavirus that erupted in Wuhan China in late 2019, and restricted air travel. This was xenophobia, political critics charged, and not based on evidence. Closing borders was not going to work anyway.

Among the early adopters of travel restrictions with significantly lower death rates are countries such as Australia, Israel and the Czech Republic. Germany, which introduced its first travel restrictions on 16 March, belongs to the late-adopter group, but countries such as Great Britain, France or Brazil responded even later. New York City refused to adopt similar measures, their Mayor even told residents to rush to bars for one last drink because, he said, the federal government was going to force him to close them, and that city ended up being much harder hit than California, which restricted travel earlier.

A new analysis of network diffusion shows that countries that are heavily exposed to international travel and tourism - such as France, Italy, and the USA - recorded significantly higher numbers of deaths. At the same time, death rates in countries more at the margins of international travel networks as well in island states remained comparatively low.

Entry bans and mandatory quarantines worked, and the earlier the better. Crucially, travel restrictions needed to be in place before the local spread of the virus had spiraled out of control. If one compares countries that imposed travel restrictions until early March to countries that implement them from mid-March onward or not at all, mortality within the first group is an estimated 62 percentage points lower than in the second group.

The study shows that the type of travel restriction also plays a role. Mandatory quarantines for incoming travelers were more effective than entry bans. A plausible explanation is that entry bans often include exceptions for both citizens and permanent residents. By contrast, quarantine measures tend to apply to all incoming travelers, regardless of their nationality or country of residence. The study further shows that targeted travel restrictions (represented in the study by entry bans and mandatory quarantines for travelers from China or Italy) were more effective than restrictions targeted against all foreign countries.