Tonga and two international health agencies are investigating tens of thousands of faulty measles vaccines.

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In November, Tonga said it would re-vaccinate up to 20,000 people after dozens of people with measles were found to have received both vaccine doses.

Ministry of Health chief executive Dr Siale 'Akauola said it's assumed the vaccines were either poor quality or were incorrectly stored.

He said the Health Ministry is investigating alongside the World Health Organisation and the US Centers for Disease Control and Prevention.

According to the latest available data, there were 440 confirmed or suspected measles cases in Tonga.

Dr 'Akauola said Tonga was confident it had 99 percent immunisation coverage for measles until it discovered people sickened with the disease who had already received both doses of the vaccine.