The study, published last month in PLOS Currents: Outbreaks, simulated the number of passengers traveling daily from West Africa to other parts of the world in an effort to quantify the risk of the disease spreading Ebola internationally.

The short-term probability of the virus spreading to countries outside of Africa, the researchers concluded, is small; however, if the current outbreak isn’t contained, the chances of it spreading internationally will increase.

The group examined 220 different countries. Initially, they found that among western countries, the U.K. was most at risk for importing Ebola. But since the paper was published, the U.S. diagnosed its first case of the disease.

Thomas Eric Duncan, a Liberian national who had traveled to Texas from Liberia, was diagnosed with Ebola on Sept. 30. He died eight days later, and since then, two nurses who cared for him have contracted the disease.

Before developing symptoms, one of the nurses, Amber Vinson, traveled on a commercial airline, to Ohio and back, potentially exposing dozens of others to the deadly virus.

Mr. Vespignani regularly publishes new projections online and now estimates that, apart from other countries in Africa, the U.S. has the greatest risk of importing additional cases of the Ebola.

As for the effect of Ms. Vinson’s travel on the possible spread of Ebola in the U.S., Mr. Vespignani said in an email that it is too early to understand the extent of this development.