Tongan health authorities could re-vaccinate up to 20,000 people against the measles after it was discovered some historical vaccinations might not be effective.
There have been at least 177 suspected cases of measles in the country since an October outbreak was declared.
Ministry of Health chief executive Siale 'Akau'ola said of those cases, 43 had a two dose recorded history of vaccination, which was recommended for protection against the virus.
"Even though some children have two doses, they still contracted the measles. That is an interesting finding. We have been trying to trace the vaccines that have been given but they were given 15-16 years ago."
He said the vaccines or systems could have been inadequate in the past but they were investigating the issue.
"We are assuming that the quality of the vaccines or the cold chain systems of vaccination at the time were not of the best quality that we have seen in the last 10 years for example."
He stressed current systems had improved.
Dr 'Akau'ola said because there had been very few cases of measles below the age of 10 it was thought recent vaccinations had worked.
The Ministry of Health had sent teams out to high schools, initially in areas where there were pockets of outbreaks, to re-vaccinate people.
Dr 'Akau'ola said even those who had recorded two doses of vaccine would be administered new shots.
He said the target group was people between the ages of 10 and 20.
The current measles outbreak was declared in October after a number of members of a high school rugby team that had returned from New Zealand developed the illness.
The Tongan Futsal team was also quarantined after travelling to a regional tournament in New Caledonia through New Zealand.
Seven members of that team developed measles, including four who had been administered two doses of the vaccine in the past.
The Ministry of Health says there is no evidence to suggest new lines of transmission, apart from those who had been to New Zealand.
Dr 'Akau'ola said most of the measles cases in Tonga had experienced only a mild illness.
Measles outbreaks had also been declared in Samoa and Fiji.
In Samoa there have been 628 suspected cases and at least seven deaths suspected to be related to the disease.