Tongans flocked to vaccination centres Saturday after the government warned the main island Tongatapu might be plunged into lockdown next week after recording its first case of Covid-19.
The infected person was among 215 people on a repatriation flight from the New Zealand city of Christchurch.
A routine test on arrival Thursday, while in compulsory managed isolation, returned a positive result the following day.
Prime Minister Pohiva Tuionetoa warned islanders Saturday to prepare for the possibility of a lockdown if more cases emerge but said there was no need for immediate action as it could take "more than three days" before a person with the virus becomes contagious.
"We should use this time to get ready in case more people are confirmed they have the virus," he said.
The tiny Pacific kingdom, about 1,800 kilometres (1,100 miles) northeast of New Zealand, had been among only a handful of countries to escape the virus that has affected billions worldwide and claimed nearly five million lives.
Only about a third of Tonga's population of 106,000, most of whom live on Tongatapu, have been double vaccinated against Covid-19.
But news that the first case had been detected in managed isolation prompted thousands to rush to get vaccinated.
"We've been to the community several times but only a few come to the site but yesterday and today was really good, really full, very best," Tonga's national immunisation coordinator Afu Tei said.
"Almost 2,000 last night and today you can see the turnout is very good."
Health officials confirmed the infected Tongan had been double vaccinated, having received the second jab in mid-October.
Tonga health ministry chief executive Siale Akau'ola said that although it would be another two weeks before the second vaccination was fully effective, "we believe the person would not get seriously ill and reach a dangerous level."
All health workers, police and airport staff who were on duty when the flight arrived have been put into quarantine as a precaution, Akau'ola said.
New Zealand's health ministry confirmed the infected person had tested negative before the flight left Christchurch, where there are only four known cases of Covid, all of them in the same household.
The repatriation flight included members of Tonga's Olympic team who had been stranded in Christchurch since the Tokyo Games.
The athletes were double vaccinated before they departed for the Olympics.