Medical practices have been told to ration the MMR vaccine to ensure the most vulnerable patients get their jabs, as New Zealand's measles outbreak continues to grow.

Since January 1 1172 confirmed cases of measles have been notified here, 970 of them in Auckland. About one in three people with measles - and more than half of under-2s - have ended up in hospital.

The spike in measles has led to a run on vaccines with many GPs running out. Some parents report being turned away when trying to get their toddlers their first jab.

There have been 160,000 vaccines given this year compared with 90,000 in the same period last year, according to the Ministry of Health.

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The ministry today advised practices to target their remaining doses to the most vulnerable patients as the country awaits another shipment this weekend.

"Children aged two years and under are more likely to be hospitalised because of measles so it's imperative they're vaccinated," ministry director-general Dr Ashley Bloomfield said. "First and foremost we need to protect our children."

Bloomfield said the immunisation expert advisory group had advised a focus on vaccinating those who were most affected by the outbreak. This would protect both them and the wider community by preventing the highly contagious disease from spreading further.

Healthcare organisations have been told to prioritise children whose shots are due - at 15 months old, or 12 months in Auckland, and 4 years old. In Auckland practices should also prioritise 15- to 29-year-olds and Pacific peoples.

Children aged up to 14 who have had no MMR vaccine should also be proactively contacted to get their first shot, the ministry said.

The next shipment of 52,000 vaccines was due this weekend and would be distributed next week.

People with measles are contagious from five days before their rash develops until five days after. Photo / Getty Images
People with measles are contagious from five days before their rash develops until five days after. Photo / Getty Images

The rate of hospitalisation in this outbreak - around 33.6 per cent - is far higher than previous outbreaks, according to figures from the Institute of Environmental Science and Research.

By comparison, in 2011 about 17 per cent of cases were hospitalised.

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It's also three times the rate of hospitalisation in the current outbreak of measles in the United States, where there have been 1241 cases since January 1, as of last week.

Just 130 of those people ended up in hospital - about 10 per cent.

In last year's outbreak in England there were 966 cases of measles confirmed for the entire year, with about 30 per cent admitted to hospital.

Measles has seen a resurgence around the world, but for a country of New Zealand's population, the magnitude of our outbreak is very high, University of Auckland vaccinologist Dr Helen Petousis-Harris said.

"We've got a really small population but over a very short period we've had more cases than these really big populations."

There were several theories about the cause. She said it was possible other countries did not have as much of a problem with the vaccination gap in young adults.

The USA has had higher vaccination rates than New Zealand for much longer. Although New Zealand now had similar rates of MMR vaccination in children, older people were still affected by historically poor rates of vaccination.

It's also thought a more virulent strain of the disease may be to blame for high hospitalisation rates here. Most of Auckland's cases were related to a single strain that arrived in March, Petousis-Harris said.

Most of Auckland's measles cases are thought to come from a single strain that arrived in the country in March, Dr Helen Petousis-Harris says. Image / Auckland Regional Public Health Service
Most of Auckland's measles cases are thought to come from a single strain that arrived in the country in March, Dr Helen Petousis-Harris says. Image / Auckland Regional Public Health Service

Dr Caroline McElnay, the Ministry of Health's director of public health, said the situation was not unique to New Zealand - significant outbreaks were also happening overseas including in Hong Kong, the Philippines, Europe, Canada, the USA and South East Asia. Australia also has cases of measles regularly.

Since 2012, all measles cases in New Zealand have originated from travellers bringing the disease from overseas, McElnay said.

Outbreaks of measles come in cycles, arriving every 5-7 years in countries with higher vaccination coverage, she said.

The number of susceptible people built up over time until it reached a level that could sustain transmission.