Thousands of Auckland teenagers and young adults are likely to be at risk of catching measles because vaccination rates were so low when they were babies.
Auckland Regional Public Health Service (ARPHS) medical officer of health Dr William Rainger has warned that those aged between 13 and 29 years old are one of the groups most affected by Auckland's measles outbreak.
The New Zealand Herald has calculated from immunisation records that each year from 1992 to 2010 in Auckland alone more than 5000 2-year-olds were not vaccinated against measles, and in 1992 the number was likely much higher.
Health experts say that while there will have been some catchup, there has been no formal campaign to raise the immunity of today's young adult population.
New Zealand children receive two measles vaccinations, the first at 15 months old and the second at 4 years old.
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.
Anti-vaxx debate: Vaccination rates plummet for NZ babies
Watch: Prime Minister gives measles outbreak update
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.