A medical expert says the trans-Tasman COVID-19 travel bubble with Australia is responsible for the RSV outbreak in New Zealand and the border shutdown during 2020 has given the virus a "double whammy" kick this year.
Cases of RSV in New Zealand's six main hospitals have been slowly rising since May, but in the past two weeks, they've more than doubled from 204 to 538 infections.
Royal New Zealand College of General Practitioners (RNZCGP) medical director Dr Bryan Betty told The AM Show symptoms are particularly bad for children under the age of two.
"The symptoms are for most children, most adults, it's very mild, just like a cold - a sniffly nose, a bit of a cough, slight temperature - that's what happens. Under the age of two and under the age of one, it can become more serious and what we start to see is a very high temperature, maybe some difficulty with breathing and going off your food, bottle or breast.
"Those three things should be a warning sign that there's a problem and you need to go and see your GP or an after hours [clinic]. For a very small number of children, it can become very serious, ending up with pneumonia or lung infection and ending up in hospital and occasionally in ICU and that's incredibly distressing for parents."
The outbreak is overwhelming hospitals including Starship in Auckland which is currently postponing surgeries to make room for the influx of infants suffering from RSV. Middlemore Hospital is also struggling with a surge in admissions, while Waitakere Hospital has reported queues snaking out its doors.
Dr Betty told The AM Show RSV comes in from overseas every year, but it didn't in 2020.
"Last year it was really interesting - we had none of these illnesses because the borders were closed so they weren't brought in by aeroplanes, they weren't brought in from overseas. We have opened the border with Australia and RSV has come back in."
But he said not having it for a year is causing many of the issues in New Zealand.
"Now the problem is, because we didn't have it last year, a lot of our very young children have had no exposure to it and it's like a double whammy. They are getting hit very, very hard this year and that's what we are seeing."
He said once you get it, you build up immunity which is why older children and adults don't get as severe symptoms as young children.
Dr Betty is urging parents with sick children to keep them home from ECE centres and schools to stop the spread of any illnesses.
"You tend to catch these viruses in close contact so we tend to see them go around childcare centres, around houses and families. It's very contagious," he said.
"What's really, really important, is if your child is unwell with a sniffle or a cough, maybe the symptoms of a cold or flu, keep them at home. That way it doesn't spread around the centre. That's probably the best piece of advice I can get with that."
The Director-General of Health Dr Ashley Bloomfield has also expressed concern about the rapid rise in cases.
"[I'm] certainly concerned about the sharp surge in RSV cases. This is a nasty illness," he said on Wednesday.
"Babies and children suffer from a whole range of illnesses and often there's no discussion about a particular virus, but certainly RSV is one that's common. It's more common that Māori and Pasifika children get sick with RSV during the winter."