With New Zealand going through its worst outbreak of mumps since 1994, three health experts say many young people are unaware they have not been immunised.
Auckland University Immunisation Advisory Centre director Nikki Turner, who is also a GP, said many adolescents and young adults missed out on the mumps vaccine when they were younger and could be unaware.
"The majority of these cases come from people who are under-vaccinated or not vaccinated at all," she said.
Dr Turner said the cases also stemmed from migrants from countries which could not afford measles, mumps and rubella vaccines.
Waikato DHB director of public health Felicty Dumble said many parents assumed their children had been fully vaccinated without being sure of it.
Ministry of Health chief advisor for child and youth Pat Tuohy said part of the problem was the medical system does notdo enough to make sure people had the vaccine when they should have.
Mr Tuohy said the establishment of the National Immunisation Register had improved immunisation rates.
The experts said anti-immunisation sentiment was not a huge issue in the communities they worked in, and movies such as Vaxxed tended to preach to the converted.