New Zealanders aged 15-to-29, the age group that helped to fuel last year's measles outbreak, will be targeted in a new government immunisation programme.
Health Minister Julie Anne Genter said it had ordered 350,000 doses of the measles, mumps and rubella (MMR) vaccine to target the age group, which has low immunisation rates.
But some in the health field said if the government had acted faster, the measles outbreak could have been prevented.
Vaccinologist Helen Petousis-Harris said the mumps outbreak in Auckland two years ago was spread among the same age group.
She remembered doctors and others with an interest in health, almost begging for a targeted MMR campaign at the time.
"[Mumps] was about the biggest clue that could have been dropped and, of course, a short time later, we did have the measles epidemic," she said.
The young New Zealanders had low rates mostly because of shortfalls in health systems and registers at the time they were children.
The director of Auckland University's Immunisation Advisory Centre, Nikki Turner, said it was frustrating the catch-up had not been done earlier.
But the more people who were immune to measles, the better protected the country would be from a new outbreak, she said.
"Measles is an international viral problem ... it's huge. We can't stop the import but what we can stop is them spreading through the New Zealand community and this is one important gap," Dr Turner said.
Genter said more than 370,000 vaccinations were given last year but the focus was not the young adults.
That was because health officials had to focus on those who were at most risk of harm, she said.
"We had to make sure there was enough for all the babies because they are the ones who are most likely to be hospitalised or seriously ill," she said.
Auckland Medical Officer of Health William Rainger said the 15-29 group was notoriously challenging to reach.
They were relatively well so did not go to GPs as much and most had left school, he said.
DHBs would probably need to work with sports clubs, marae and other community groups to find them, he said.