A National Immunisation Register was created in 2005. Prior to that, immunisation levels were estimated based on vaccination claims from general practitioners and infrequent national coverage surveys.
Records from the immunisation register show that in 2009, the first year data is available, only 77 per cent of those who turned 2 in Auckland were fully vaccinated. That means 5517 young children in that year were not fully vaccinated.
The Herald has calculated how many children were not fully immunised at age 2 in each of Auckland's [current] local boards in 2009.
This data suggests while the majority of current measles cases are in Counties Manukau, there is still an immunity gap in the rest of Auckland.
Dr Nikki Turner, the director of the Immunisation Advisory Centre, says we don't know how big the immunisation gaps are and even if many people received catch-ups, a lot would still have missed out.
Blood tests for immunity are also used to estimate immunisation gaps. New Zealand has done these, but Turner says they don't give us any extra data and they suggest that 20 to 30 per cent of the young adult population might not be immune to measles.
Turner's research shows that the 2009 2-year-olds are likely the best protected of today's 13 to 29 years olds. In 1992, 2-year-old vaccination coverage may have been below 60 per cent, meaning almost 10,000 Auckland infants weren't fully vaccinated at that age.
Low vaccination rates have been blamed on the anti-immunisation movement and misplaced fear about links between autism and the MMR (measles-mumps-rubella) vaccine from a discredited study in the 1990s.
But Turner said the data showed many people didn't get their children vaccinated for other reasons, such as difficulty accessing health services, and they were not necessarily anti-vaccination.