Travel restrictions imposed as the new coronavirus took China by storm slowed the spread of COVID-19 by only a few days within China and a few weeks internationally, according to a new study.

On January 23, Chinese officials shut down travel in and out of Wuhan, the city where the COVID-19 outbreak began, including closing the airport. But by then the virus, called SARS-CoV-2, had already spread to other cities in mainland China. As a result, the travel ban delayed the progression of the outbreak within the country by only three to five days, researchers report online March 6 in Science. The study simulated the impact of restricting travel on the spread of COVID-19 using population data, travel patterns and disease transmission.

While minimally effective within China, the Wuhan travel ban initially had a larger impact on international spread of the virus. The simulation suggested that there were 77 percent fewer cases imported from mainland China than would be expected absent the ban. But starting in mid-February, the number of international cases rose as other places in China where the virus had become established, including Shanghai and Beijing, began to fuel its spread to other countries.

In February, 59 airlines stopped or curtailed flights to mainland China. It’s difficult to pinpoint how much travel was reduced, but the researchers analyzed the potential impact of a 90 percent drop in overall traffic to and from China. On its own, this scenario could slow the progression of the epidemic by only a matter of weeks.

Despite the extensive travel restrictions, the study suggests that many individuals exposed to SARS-CoV-2 have traveled undetected. Only when combined with robust measures to control infection in the community — such as the quick diagnosis and isolation of new cases — would such a reduction of traffic make a meaningful difference, the researchers report.