An Auckland teenager was shocked to find she had measles despite being fully vaccinated.
She's one of at least 64 people who contracted the highly contagious disease this year even though their MMR jabs were up to date.
A bad migraine and persistent fever were the first clues, the 18-year-old told the Herald.
"The headache just did not go away. I had really bloodshot sore eyes...and usually Nurofen works like a dream," said the teen, who asked not to be named.
"I looked at the symptoms online and it came up with measles. But I was like, nah I've been vaccinated so probably not."
The Ministry of Health says although MMR is highly effective against measles, a few people will not have an immune response to the vaccine.
But if enough people are immunised, those few will be protected as they are unlikely to come into contact with the disease. This is known as herd immunity.
While the teen has now recovered, her mum told the Herald she feared the disease could have turned deadly.
"My big concern is that as a parent I've done my job by getting my child vaccinated... and now she's contracted it," she said.
As of last Friday, 64 of the 1111 confirmed measles cases in New Zealand were in people who were fully vaccinated.
That's about one case in 18.
That compares to 554 cases in people who were not vaccinated at all, according to the latest weekly report from the Institute of Environmental Science Research. The vaccination status of another 465 people was not known, and 28 were only partly vaccinated.
But medical experts say 1 in 18 isn't as high as it sounds. There are far more vaccinated than unvaccinated people in New Zealand, which skews the numbers.
The MMR vaccine provides about 97-99 per cent protection against measles, a spokesman from the Immunisation Advisory Centre said.
That meant a small number - between 1-3 per cent - of vaccinated people would still get measles if they were exposed to it.
"If instead you are unvaccinated and encounter measles, then you have about a 90 per cent chance of catching it," he said.
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University of Auckland vaccinologist Dr Helen Petousis-Harris said because most people were vaccinated, there was a much higher chance of seeing those few people in whom the vaccine failed.
"The pool of people who are vaccinated is really big, and the pool of people who are unvaccinated is quite small."
As measles was so contagious, for people who were not immune the "hit rate" was extremely high.
"If you had a party with a whole lot of people in the room, if someone's got measles it's almost certainly going to find those who are not immune."
As of Thursday there were 1214 confirmed cases across New Zealand, with 1007 in Auckland.
Rates of MMR vaccination have quadrupled in the last three weeks, according to the Auckland Regional Public Health Service's medical officer of health, Dr William Rainger.
That has led to a shortage of doses in some areas due to distribution problems. Rainger said Auckland's DHBs were leading regular stock takes and ensuring MMR stocks were moved to where they were most needed.
Medical practices have been told to prioritise getting children under 5 vaccinated according to the immunisation schedule, as they are the most likely to get sick.
Pacific peoples and young people aged 15-29 are also a focus as they have been more affected by the outbreak. Getting those groups vaccinated will help stop the spread of the disease, Rainger said.
Another 52,000 doses of MMR are set to arrive in New Zealand this weekend.
MMR by the numbers
• The MMR vaccine is 97-99 per cent effective.
• Put 500 teenagers on an island. At a 95 per cent vaccination rate, we'd expect 475 to be vaccinated and 25 unvaccinated.
• If someone with measles visited the island and there was a dance party, we'd expect 14 of the 475 vaccinated and 23 of the 25 unvaccinated to get sick.
• That means 38 per cent of the teens who caught measles would have been fully vaccinated - but that group was far bigger to start with.