The mumps outbreak in Auckland has now seen 600 people affected and could continue for up to a year, says a public health expert.
In the past 18 days another 143 cases have been reported.
In early September about 300 cases had been recorded by the Auckland Regional Public Health Service during the year and by September 28 that had jumped to 457. On October 9, it hit 554 and as of today there had been 600 cases reported.
Last year only 13 cases were reported throughout the city.
Auckland Regional Public Health Service medical officer of health Josephine Herman said she believed the outbreak was hitting its peak at the moment and the numbers would begin to decline - although the outbreak could continue for another year.
"I think it's a bit early to determine where this is going," she said. "We have been looking at some models to see if we can estimate or predict how long it will last for... At the moment we're thinking this might go on for another year. Once we have peaked we are likely to have fewer cases."
She said it was difficult to predict what would happen because vaccination coverage data had only been well collected for the past 12 years and this outbreak was affecting 10 to 29-year-olds the most.
Most of the reported cases had been mild but about 5 per cent had serious symptoms which required hospitalisation. Many of those serious cases involved orchitis - the painful swelling of the testes, she said.
The majority of the 600 cases had been in South and West Auckland.
The last outbreak was in 1994, a few years after the measles, mumps and rubella (MMR) vaccine was introduced to New Zealand. Before then, there were outbreaks every three to four years.
Herman said the outbreak may have spread so far because many children missed out on their second vaccination in 2001 when the age of the second jab was changed from 11 to 4.
While the outbreak had affected cities and towns across the country it was worst in Auckland because of the city's population, she said.
She said there were likely to be many more cases that had not been reported because in about 30 per cent of cases people had no symptoms.
The only solution to the outbreak was to continue to push vaccinations, Herman said.
In the Auckland region just above 90 per cent of 2-year-olds had their first MMR vaccination and about 89 per cent of 5-year-olds had received their second shot.
Studies suggested vaccination rates needed to be as high as about 95 per cent to prevent outbreaks, Herman said.
"We still have a way to go," she said. "I'm hoping we can get more people vaccinated so we can help get this outbreak under control as soon as we can."
She stressed that MMR vaccinations were free for anyone who had not had their two shots and encouraged those who were unsure to contact their doctor.
Mumps - The symptoms
• Early symptoms of mumps include fever, headache, muscle aches, tiredness, and loss of appetite. The salivary glands on one or both sides of the face, cheeks or jaw may become swollen and sore after two days.
• Men and adolescent boys can experience pain and swelling in their testicles, which in rare cases can result in infertility. Females can experience ovarian inflammation.
• For pregnant women there is risk of miscarriage in the first three months.
• In some people mumps can cause permanent hearing loss.
• In very few cases, mumps can lead to inflammation of the brain and surrounding tissue (meningitis).