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

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. Officials say the individual tested negative before traveling. The traveler was asymptomatic when tested, according to the news release, and will continue to be monitored.

“The discovery of this positive case during quarantine highlights the importance of why our process is critical to prevent the spread of COVID-19,” Gov. Lemanu Mauga (D) said in a statement. “It further highlights the importance of maintaining our current quarantine protocols.” Mauga was among the passengers on the Monday flight from Hawaii and is in quarantine, according to the Associated Press.

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. The “covid zero” approach, in which countries such as Australia mandated lockdowns, hotel quarantines and other measures to stamp out transmission, probably is not possible in the long term, he said.

“Can any nation, entity, territory, forever and ever evade coronavirus? I think that answer is almost for sure no,” Kelen said. Noting that Australia has had outbreaks despite its early success, he added, “Think of it almost like a pressure cooker. As soon as you release the valve just a little, it’s over.”

The infected traveler was an America Samoa resident who had traveled to Hawaii and to the U.S. mainland, according to the Associated Press. The passenger returned Monday, the first day that commercial flights from Honolulu to Pago Pago had resumed since last year.

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. He said it’s possible the traveler was “exposed at any time traveling from the mainland, and the time that they were in Hawaii.”

“It’s very hard for us to pinpoint exactly where the exposure happened at this point,” he said. “But we know for sure that the processes we have in place are working. This passenger, their positive result was caught in quarantine.” Officials said the person was moved to isolation.

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.

“It does show we cannot let our guard down,” Chen said, adding, “Testing is still important, even with full vaccination.” She said even the fully vaccinated should think about getting themselves tested before and after travel even if it’s not required by the destination.

In a post in late August, the American Samoa Health Department posted an infographic on social media, outlining its travel guidelines and noting that it was monitoring travelers. It notes that three negative PCR test results are required before departure, at 10 days and five days before departure as well as 48 hours before.

Hawaiian Airlines announced this month that it would resume nonstop flights between Honolulu and the U.S. territory on Monday, service that had been suspended since March 2020.

The airline, which said in a statement that it “provides the only regularly scheduled air link between the two island chains,” had paused the service for 17 months at the request of the American Samoa government. Repatriation flights had resumed this year to bring thousands of residents to American Samoa who had been stranded elsewhere during the pandemic, the airline said.

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. Several “zero covid” claims are viewed with skepticism. Many experts doubt North Korea would acknowledge an outbreak, and reports suggest that Turkmenistan, another country that denies having coronavirus cases, is experiencing a wave of infections, according to BBC News.

Chen said the new infection in American Samoa may suggest the coronavirus is present in other places that have yet to report cases. “It may be circulating, it may be there already, and we just don’t know it yet,” she said.

Kelen said immunity — either through a huge number of people being vaccinated or the virus moving “through a lot of people” — is the only way out of the pandemic. He said that it is too difficult to contain, and that borders cannot stay closed forever. “No country can lock this thing down,” he said.

In American Samoa, just over 48 percent of the population is fully vaccinated against the coronavirus, compared to nearly 55 percent of the total U.S. population, according to tracking by The Washington Post.