Parents are being warned against "chickenpox parties" because the highly infectious disease can be fatal, or lead to severe complications and permanent disabilities.
Medical officer of health for Waikato District Health Board Dr Felicity Dumble said advice to put healthy and infected children together was misguided and dangerous and not recommended by health authorities.
The warning comes as one mother calls for chickenpox vaccine to be added to the immunisation schedule after her daughter almost died of complications from the illness.
And recommendations from experts to fund the varicella vaccine are growing stronger as 50,000 people are infected with chickenpox each year. More than 400 have to go to hospital because of complications, and up to two people die.
It follows the addition of the varicella vaccine to the schedule this month for children and adults at high risk of disease because of other medical conditions, but the general population must pay $70 to be immunised.
Dr Dumble said the idea of chickenpox parties to spread the illness was worrying.
"We do not endorse or recommend that because while for most it can be a mild illness there's always that risk of complications, and how would people feel if they'd done that and the child got seriously ill and ended up in hospital?"
Michelle Bibby grimaces every time she hears about a chickenpox party after her daughter Isla ended up critically ill in Starship hospital in January 2010 with pneumonia. She also suffered fluid and inflammation of the pericardium sac and an enlarged liver from chickenpox complications.
The virus lowered her immunity so drastically the 2-year-old developed a severe Staphylococcus aureus infection, causing the pneumonia and resulting in risky heart surgery.
"I would wake up every morning and think 'Is she dead?'," recalls Mrs Bibby, who wants varicella vaccine on the schedule.
The little girl was almost in heart failure with 250ml of fluid compressing her heart, and a liver five times larger than it should have been.
Surgeons found blisters filled with pus fused to her heart and had to scrape away the pericardium sac and douse the heart in antibiotics.
Isla's case was so extreme it was documented in the New Zealand Medical Journal where specialists questioned whether it was time for a universal chickenpox vaccine.
Her severe complications could have been avoided or vastly reduced if she had been vaccinated, even after initial exposure.
But the family were not advised to have the vaccine which has been a scheduled immunisation in the United States since 1995 and in Australia since 2005, where it has steadily reduced cases of the illness and hospital admissions.
Wellington GP and Immunisation Advisory Centre director Dr Nikki Turner said chickenpox parties made sense before a vaccine was available.
"But now we have a good effective vaccine, why on earth would you do a chickenpox party?
"I think it's incredibly inappropriate to do that now. I think people who have got some fears about vaccines do that because it doesn't make any common sense to have a chickenpox party."
Dr Turner, who is on a national advisory committee to medicine funding agency Pharmac, said the committee had been recommending varicella vaccine for universal inclusion on the schedule for many years.
A Plunket clinical adviser, Allison Jamieson, said chickenpox parties were not as common as in past decades and Plunket advised against them.
"You don't know what other conditions children have that might compromise them if they got an illness and some of the complications of those illnesses are terrible."
I was told the sooner they got it the better, says mum
Helen Furneaux took her children on play dates with their chickenpox- infected friends to deliberately expose her sons to the illness.
The three boys, then aged 8, 5 and 18 months, all caught the virus at the same time, probably from one of the play dates.
"If I knew of people whose kids had chickenpox it didn't worry me to go round or to have them at my place but I knew the sooner my kids got it the better," said Mrs Furneaux.
"I was always taught the sooner they have it the better because if they get it later on it could be worse."
The 38-year-old Hamilton mother-of-four said fortunately her three older boys, Eli, 10, Matthew, 7, and Liam, 4, were only mildly affected by the virus, though they have a few minor scars.
"They got it on the legs, the tummy, the back, the face, and on their head.
"They ended up with three or four little indents," she said.
If she had known three years ago how serious the complications could be, Mrs Furneaux said she would not have deliberately exposed the boys.
"Had I known about the complications ... I think I would have got them vaccinated.
"I'd never heard of anyone who'd had major complications."
She had vaccinated all her children, including youngest Samuel, now 2, with all infections covered on the New Zealand immunisation schedule.
Mrs Furneaux said Samuel had not yet caught chickenpox and she was likely to pay for him to have the varicella vaccine.