Little Ryder Passi is just 16 months old but he and his mum could be helping save hundreds of Kiwi kids from being hospitalised each year.

Rebecca Passi is one of more than 100 pregnant New Zealand women taking part in an international trial to test a vaccine's effectiveness against RSV (respiratory syncytial virus), which causes respiratory infections ranging from colds to bronchiolitis and pneumonia.

RSV is thought to cause 40 per cent of admissions to hospital for under-2s with chest infections in New Zealand. It is most severe in babies under 6 months. Tens of thousands of children under 5 worldwide are estimated to die each year from respiratory infections caused by the virus.

Passi, 39, volunteered for the RSV trial in its first year in 2015 when she was pregnant with Ryder. Older son Kobe, 4, was hospitalised twice at just 1 with bronchitis.

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"That was raising alarm bells and I didn't want my younger one to go through the same thing," she said.

One episode was particularly concerning. She rang a GP surgery at about 11pm when Kobe's breathing was "out of sync ... gasping for breath almost".

The after-hours nurse asked her to place the phone close to her baby son so she could listen to his breathing.

"In a flick of the switch, she was ringing [the] ambulance to come out ... It was pretty scary."

Passi, who also has daughters age 17 and 10, was motivated too out of concern for the health of other children in the Counties Manukau area, which is particularly hard hit by RSV.

Kidz First children's hospital doubles staffing in winter to deal with respiratory infections, consultant paediatrician Dr Adrian Trenholme said.

RSV is most prevalent in New Zealand from early May to October with peak infection rates in July and August.

More than 1000 children under 2 are hospitalised each year with bronchiolitis and pneumonia in the Counties Manukau area, he said. One in 20 end up in ICU.

"We are known as the RSV capital of Australasia," Trenholme said.

Passi, from Papakura, and who works at Bereavement Care Services at Middlemore Hospital, was motivated to take part in the trial because she had "been brought up in Counties Manukau" and was aware "sickness had a huge effect on our children".

She wanted to raise awareness of RSV, and be part of a project that might lower its incidence.

This trial is being performed worldwide including Australia, South Africa, the Philippines, Europe and the United States, involving thousands of women and is expected to provide results within three years. In New Zealand, women are being trialled in Auckland, Christchurch and Wellington with further centres to be involved.

Women are carefully screened and have to have healthy normal pregnancies. The vaccine is given after 28 weeks of pregnancy. Mothers transfer antibodies but not the vaccine to the infant, with the aim of protecting against RSV for the first few months.

Trenholme, a principal investigator for the trial, said safety monitoring was vigorous and there had been no serious side effects for mothers or babies.

As is normal for all vaccine trials, some women receive a placebo and some the active vaccine. The women and researchers do not know who gets the vaccine so that assessment of effect and side effects is unbiased.

Passi said she had been well informed before taking part in the trial.

"I wasn't hesitant in putting my hand up."

She had experienced no side effects and had remained in good health, she said.

Ryder had been well since birth apart from a bout of bronchopneumonia at the beginning of this year.

RESPIRATORY SYNCYTIAL VIRUS (RSV)

Causes:

The virus spreads through tiny droplets that go into the air when a sick person blows their nose, coughs or sneezes. It can live for up to five hours on countertops and for several hours on used tissues.

Symptoms:
Includes a runny nose, coughing, wheezing and breathing difficulty which when severe makes breathing and feeding difficult and is very distressing for infants.

Treatment:
Is only supportive as antibiotics are not effective and medicines for asthma don't work. Fluids are given by tube or IV oxygen where needed and support of breathing in intensive care for the most severe.

Outlook:
Full recovery takes about three weeks but the disease normally gets worse for three days before getting better.