Vanuatu reported its first Covid-19 case Wednesday, ending the Pacific island nation's status as one of the few remaining virus-free countries in the world.

Health officials said a 23-year-old man who recently returned from the United States had tested positive on Tuesday while in quarantine.

As the rest of the world struggled to contain outbreaks, Pacific island nations swiftly isolated themselves, despite the economic cost, fearing their poor health infrastructure made them particularly vulnerable.

Vanuatu closed its borders in March in a bid to keep the pandemic at bay, only recently allowing in strictly controlled repatriation flights.

"I want to assure the public and citizens of this country that this situation is under control," Prime Minister Bob Loughman told the nation of 300,000 people.

The infected man, originally from Vanuatu, had travelled from the United States via Sydney and Auckland, but had been isolated from other passengers during the journey as a precaution.

Loughman indicated that -- because the man was still in quarantine and contact tracing was under way -- he would not put restrictions on public gatherings, close schools or ask people to work from home.

But some domestic travel would be restricted, the country's mandatory quarantine for repatriates was set to be doubled to 28 days, and citizens would have to show negative test results at most 72 hours before departure.

"The timely and hard closure of Vanuatu's borders has bought them crucial time to plan and act strategically," Australia-based public health expert Lana Elliott told AFP.

The government's actions, she said, "kept their population safer than just about any other country on earth".

The Solomon Islands and Marshall Islands lost their virus-free status last month, although, like Vanuatu, they have so far avoided community transmission.

The remote island nations and territories of Kiribati, Micronesia, Nauru, Palau, Samoa, Tonga, and Tuvalu are believed to still be free of the virus.