“Even countries that have been relatively successful in managing the pandemic are only as safe as the weakest links in the system,” she said, adding that in the absence of cooperation among countries, “closing borders is one of the ways that individual governments can control the situation.”

The virus, which emerged in Asia and spread to the West, is at risk of ricocheting back. Citizens who were worried about outbreaks in Europe and the United States rushed home after finding themselves in the new epicenters of the pandemic.

Almost immediately, countries and cities in Asia started seeing a rise in new cases, often detecting infected passengers at airports as they passed through health screenings. Hong Kong, which had been reporting new daily cases in the single digits, suddenly saw new cases spike as high as 65 in one day. In Japan, where infections have remained relatively controlled, cases started to rise last month in Tokyo as travelers returned from overseas.

To try to stem the influx of infections, governments clamped down on their borders.

South Korea, which has been praised globally for flattening the curve quickly after an early explosive peak in infections, initially required travelers from some countries to quarantine. This week it expanded the list to cover the entire world.