American Samoa reported its first coronavirus case, 18 months into the pandemic, after a traveler tested positive after flying to the U.S.

The positive case was discovered during a quarantine period required upon arrival in American Samoa.

The traveler was fully vaccinated, according to a news release published Friday by American Samoa’s Department of Homeland Security, and the positive result was confirmed by the Health Department Thursday.

Gabor Kelen, director of Johns Hopkins University’s Department of Emergency Medicine, told The Washington Post it was surprising that American Samoa was able to evade the coronavirus as long as it did.

According to the World Health Organization coronavirus tracker last updated Friday, American Samoa had reported no confirmed coronavirus cases, but it was not clear why that was so.

In a news conference on Friday, American Samoa Health Department epidemiologist Aifili John Tufa said the traveler had tested negative before flying back home.

Lin Chen, director of the Travel Medicine Center at Mount Auburn Hospital in Cambridge, Mass., and an associate professor at Harvard Medical School, said the positive report in American Samoa shows that protocols like testing and quarantine upon arrival can pick up positive cases, even if tests before departure were negative.

The report of American Samoa’s first positive case leaves a dozen countries, regions and territories that had reported no coronavirus cases, according to the World Health Organization.

Chen said the new infection in American Samoa may suggest the coronavirus is present in other places that have yet to report cases.