For more than two years, the isolation of the Pacific archipelago nation of Tonga helped keep COVID-19 at bay.

But last month’s volcanic eruption and tsunami brought outside deliveries of desperately needed fresh water and medicine — and brought the virus.

Tonga is only one of several Pacific island countries to experience their first outbreaks over the past month. There is growing concern that their precarious health care systems might quickly become overburdened, and that the remoteness that once protected them may now make them difficult to help.

“Clearly when you’ve got countries that have already got a very stretched, and fragile health system, when you have an emergency or a disaster and then you have the potential introduction of the virus, that’s going to make an already serious situation immeasurably worse,” said John Fleming, the Asia-Pacific head of health for the Red Cross.

Tonga was coated with ash following the Jan. 15 eruption of the massive undersea Hunga Tonga Hunga Ha’apai volcano, then hit with a tsunami that followed.

Only three people have been confirmed killed, but several small settlements in outlying islands were wiped off the map and the volcanic ash tainted much of the drinking water.

Also read: Virus enters Tonga along with disaster aid, lockdown planned

The nation of 105,000 had reported only one case of COVID-19 since the beginning of the pandemic — a Church of Jesus Christ of Latter-day Saints missionary returning to the island from Africa via New Zealand who tested positive in October — and authorities debated whether to let international aid in.

They decided they had to, but despite strict precautions unloading ships and planes from Australia, New Zealand, Japan, Britain and China, two Tongan men who worked at the capital’s Queen Salote Wharf handling shipments tested positive on Tuesday.

The two were moved into isolation, but in tests of 36 possible contacts, one’s wife and two children also tested positive, while the others tested negative, the local Matangi Tonga news site reported.

“The highest priority now is the close contacts of these positive cases and whom they interacted with” since Jan. 29, Health Minister Saia Piukala was quoted as saying.

It was not clear how many people they might have come into contact with, but the government released a list of locations where the virus might have spread, including a church on two days, several shops, a bank on two days and a kindergarten during school hours.

Prime Minister Siaosi Sovaleni told reporters “some” of the five infected had started to show symptoms and were being quarantined at a medical facility. He imposed an open-ended lockdown starting 6 p.m. on Wednesday; one that could be particular arduous for Tongans because most have been without any internet connections since the volcanic eruption severed the only fiber optic cable to the country.

It is not yet known what variant of the virus has reached Tonga, nor who brought it in.

Sailors aboard the Australian aid ship HMAS Adelaide reported nearly two dozen infections after an outbreak on board, but authorities said it had been unloaded at a different wharf. Crew members aboard aid flights from Japan and Australia also reported infections.

Experience from elsewhere in the pandemic, especially with prevalence now of the rapid-spreading omicron variant, suggests that Tonga faces an uphill battle in trying to contain the outbreak, Indonesian epidemiologist Dicky Budiman told The Associated Press.

Also read: 23 Australians on ship delivering aid to Tonga have virus

Some 61% of Tongans are fully vaccinated, according to Our World in Data, but because the country has not yet seen any infections, there will be no natural immunity and it is not clear whether the shots were given long enough ago that they may now be less effective, Budiman said.

The global experience has been that when the virus hits such countries, the number of hospitalizations is very high, he said, recommending that the government immediately start offering booster shots and open vaccinations to younger children.

“If we race with this virus we will not win,” he said in an interview from Australia. “So we have to move forward by protecting the most vulnerable.”

This is already starting to be seen in the Solomon Islands, which reported its first community outbreak on Jan. 19. With only 11% of its population fully vaccinated, the virus has been spreading rapidly with the Red Cross reporting that less than two weeks later, there are now more than 780 recorded cases and five COVID-19 related deaths.

Elsewhere, Fiji — still reeling from damage caused by Cyclone Cody in early January — has been battling an ongoing spike in cases, fueled by omicron, and cases have been reported for the first time in Kiribati, Samoa and Palau.

Palau has nearly its entire population fully vaccinated, while Fiji has 68% and Samoa 62%, but Kiribati is only at 33%.

The key to ensuring hospitals aren’t overwhelmed is to make sure more people get shots, Budiman said.

“These countries that choose to have this COVID-free strategy, they are very vulnerable,” he said.