Auckland's measles outbreak is leading to many more people than usual - about 50 percent of those diagnosed - ending up in hospital, the regional health service says, and they do not know why.

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About 10 percent of people with measles usually end up in hospital but Auckland Regional Public Health Service clinical director Julia Peters said of the measles cases confirmed in Auckland to date this year, half had ended up at hospital.

"Some of the information we've had is that people do seem to be more unwell in this outbreak and therefore more people with the measles are ending up in hospital," she said.

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"However, it's also possible that within these numbers there are people that are being kept in the emergency department for a bit longer than maybe in the past, potentially for individual investigations or observation and then are crossing a definitional line - if you like - and then being counted as a hospitalisation but not actually being admitted to a ward."

Dr Peters could not give precise figures for how many would fall into that category, and they did not know why it seemed to be so high.

She said children and teenagers were the most at risk.

"The most affected groups are children under four years of age, and those aged 13 to 29 years. Children and infants under to four years old, they're making up 38 percent of the cases and then 15 to 29-year-olds: 30 percent of confirmed cases.

"In the Auckland region a third of our confirmed cases are Pacific, 43 percent European, 15 percent Māori and the balance are Asian."

Dr Peters said it did appear more people were ending up in hospital with more severe cases.

"Measles is a very nasty illness. People who get the measles feel extremely unwell; people who end up in hospital are likely to have a much more severe illness ... they may have an associated respiratory tract infection so, for example, pneumonia. As in the case in Northland, they may have some inflammation around their brains (encephalitis) although that is quite unusual.

"They may have a very high fever, they may be very lethargic and dehydrated and need IV fluids ... so there can be a range or reasons people can end up in hospital."

She said it was important for people to check their records and make sure they were immune.

"I wouldn't say it's out of control but I think our public health efforts to stamp it our are clearly not being as effective as we'd like them to be and therefore we really do need to encourage people to think about their vaccination status and whether they are immune to measles. Get out those Well Child books that your mother kept and check either in there or with your GP that you are vaccinated against measles."

It was also important for people who were worried they had been infected to reduce the risk of infecting others.

"If you think you've been in touch with someone with measles or you think you are developing symptoms of the measles then you need to see your general practitioner, but call ahead first so that when you go into your primary care provider you can be isolated straightaway."