Medical authorities in Christchurch say it could take weeks to find out whether a measles vaccine has been ineffective.
There are 15 confirmed and six probable cases of measles in Christchurch, including six people who have been fully vaccinated.
The Canterbury medical officer of health, Dr Cheryl Brunton, says they're trying to find out whether those people were properly vaccinated or if there were problems with the vaccine.
But as most of the vaccinations would have been given 10 years ago, and not all of them took place in Christchurch, tracing the records could take days or even weeks.
With the exception of a person in their early 20s, those affected are aged between 11 and 16. Twelve are from Christchurch Boys' High School, where the outbreak surfaced. There are now cases at other schools in the city.
Four people who were hospitalised have all been discharged.