Auckland's hospital admission rate for the highly contagious measles virus is up to four times more than expected, with Pacific people disproportionately affected.
Auckland's Regional Public Health Service says there have been 1307 confirmed cases of measles in the region, with South Auckland the worst affected area.
South Auckland has a large Pasifika population.
Numbers show Pasifika are about seven times more likely to catch the virus. An interim report on the Auckland measles outbreak this year to mid-September shows the ethnicity breakdown with Pasifika much more vulnerable (2.2 cases per 1,000 population), as are Māori (1.3 per 1,000 population) compared to NZ European/Other (0.3 per 1,000 population) and Asian (0.2 per 1,000 population).
The measles outbreak might have reached its peak already but the hospital admission rate is up to four times more than expected.
"The current measles outbreak is the largest in two decades in New Zealand and the proportion of people hospitalised in this outbreak, at 35-40 percent, is higher than anticipated (usually about 10 percent of cases are hospitalised)," the report said.
The authority's report analyses hospitalisations from the measles outbreak, to try and work out who is getting sick and what complications the disease is causing.
Medical Officer of Health William Rainger said hospitalisations from potentially serious complications of measles were a worry with more admissions during this outbreak than in the past.
Dr Rainger said family doctors continued to see cases from all across Auckland, but the majority of new cases were still in South Auckland.
The age distribution pattern has been consistent, with measles mainly seen in children under five years-old and he was hopeful the worst was over.
"The greatest number of cases in any given week was in the last week of August. So we're still seeing fewer cases per week than that week in August. So that supports us continuing to be cautiously optimistic that we may have reached the peak of this outbreak."
But Dr Rainger said the lag effect of the school holidays and the two week incubation period of virus transmission may yet cause a further spike in numbers.
Karen Bartholomew, director of Health Outcomes for the Waitematā and Auckland district health boards, said the current Auckland outbreak was the largest in 20 years.
Dr Batholemew said normal hospitalisation rates were predicted at 10 percent but during this outbreak 35 to 40 percent of people with measles were ending up in hospital.
"The hospitalisation rate was much higher for those aged zero to four, which we did expect, and that's 52 percent, so certainly much higher. Hospitalisation rate for Maori was 41 percent, and for Pacific was 37, so also higher for those groups. And Counties Manukau had the highest number of hospitalisations and that was expected also."
Dr Bartholomew said the high admission rate appeared to be partly because more measles cases were in young children under 4 years-old.
All cases needing hospital care were considered 'serious' by those analysing the data, but 22 percent were categorised as 'further complicated' and needed a longer hospital stay and sometimes intensive care, she said.
Sixty five people became sick with pneumonia from their measles infection and three of those patients also developed encephalitis, with these rates higher than expected number for both conditions. There were no deaths from measles complications.
However, five pregnant women were hospitalised in Auckland with two foetal deaths potentially related to maternal measles.
Dr Rainger said stillbirths, miscarriages and low birth weight, were all possible complications of measles in pregnancy.
It was understandably worrying for pregnant women in Auckland.
"But I want to make it quite clear that the likelihood of a woman who's pregnant being exposed to measles and their risk of developing measles is no greater than any other person in the Auckland population. And so any individual is at relatively low risk of actually developing measles."
Dr Rainger said all expectant mothers were routinely tested for rubella and if they were immune to that, they were also likely to be immune to measles.
Any pregnant women concerned about measles are asked to seek advise and care from their lead maternity provider and family doctor.
Babies under six months, pregnant women, and people with compromised immune systems, are not able to be vaccinated, and they rely on 'herd immunity' from others being vaccinated.
Another 100,000 doses of MMR (Measles Mumps and Rubella) vaccine are expected to arrive into New Zealand soon.
People who went to the memorial service for Tongan Prime Minister 'Akilisi Pohiva two weeks ago had been advised to be vigilant for symptoms of the highly infectious disease.
Meanwhile, the World Health Organisation is concerned the New Zealand outbreak may transmit to the Pacific Island nations and it warns travellers to get immunised two weeks prior to travel if unsure of their vaccination status.