What is RSV and what's driving New Zealand's winter outbreak? Can it be treated? Here are five things we know about respiratory syncytial virus.
Despite a winter surge that's swamped hospitals, respiratory syncytial virus (RSV) isn't at all new to New Zealand.
In fact, it's so common that most children have been infected with the virus by age 2.
Admissions to hospital with this seasonal nasty, for instance, are far more common than influenza.
Known to cause infections of the lungs and respiratory tract, RSV's symptoms are usually mild and typically mimic a common cold, but they cause a severe infection in babies - especially premature infants - and the elderly or those with weak immune systems.
New Zealand's Health Navigator website lists specific symptoms such as a runny nose, coughing, sneezing, fever, wheezing and not feeling like eating – and these usually appear in stages and not all at once.
In very young infants with RSV, the only symptoms may be irritability, less activity and breathing difficulties – but the virus can also cause serious illness, including bronchiolitis and pneumonia.
Like many other respiratory illnesses, Māori and Pasifika children are disproportionately affected, with three to five times as many admissions as other groups.
People of any age can get another RSV infection, but infections later in life are generally less serious.
People at highest risk for severe symptoms include premature babies, young children with congenital heart or chronic lung disease, young children and adults with compromised immune systems, as well as older adults, especially those with underlying heart or lung disease.
Last year, New Zealand's Covid-19 lockdown and other interventions squashed rates of RSV down by 98 per cent, compared with normal seasons.
This season, the trend has gone in the opposite direction, with experts warning that hospital visits for flu-like illnesses are skyrocketing due to babies and children having weaker immune systems from lockdown.
Hospitals across the country are postponing surgeries and creating extra bed space for children, and many babies have been ending up in intensive care units, or needing oxygen to help them breathe.
ESR virologist Dr Sue Huang said weekly numbers reported by the national virus laboratory network started climbing sharply in June, reaching more than 500 cases for the week ending June 27.
"At the moment the rates of RSV are still shooting upwards and have yet to reach a plateau."
ESR's laboratory-based virology report for January 1 to June 27 taken from the six hospital laboratories based from Auckland to Dunedin, plus the national ESR virology laboratory, reported only 20 sporadic RSV cases until late May, when weekly numbers started growing.
By mid-June the numbers reached over 100 a week, and then 538 two weeks later.
To date, 969 RSV cases have been reported in just five weeks by ESR's laboratory network, compared with an annual average of 1743 cases during the winter period reported in the five years before Covid-19.
"The exponential increase is very sharp," she said.
"The absence of RSV last winter meant there is a young cohort of children from last year, plus a new cohort this year, who have not been exposed to the seasonal virus."
While there were sporadic cases previously, Huang said the increase of national RSV cases came shortly after New Zealand re-opened the borders with Australia in late-April.
ESR and Otago University virologist Dr Jemma Geoghegan said Australia had also seen a large outbreak.
"A lot of their pockets of cases seemed to be out of season as well. Then, as soon as the bubble opened, we saw this rise in cases here."
Geoghegan said that transtasman link made sense, given the large amount of people who would have been visiting family members in both countries.
"Until we've analysed the genomes, we can't say this for certain, but that's what we hypothesise."
Geoghegan said genome sequencing was now underway, with ESR scientists working with Australian counterparts to get a firmer picture on how the virus may have travelled.
There's currently no readily-available vaccine for RSV - although scientists are exploring one - nor is there any specific treatment for it.
But most RSV infections go away on their own in one to two weeks, and parents can help their children by ensuring they rest, drink plenty of fluids and use a humidifier to increase air moisture.
Symptoms can also be treated with medicines such as painkillers, nose drops or sprays, cough syrups and drops, throat lozenges and decongestants.
But parents are advised to talk to their healthcare provider before giving their children cold and cough medicines, as some contain ingredients that are inappropriate for children.
For infants and children who are at high risk of severe illness, such as babies born prematurely or with congenital heart disease or chronic lung disease, a medicine called palivizumab, and given by injection, can prevent severe RSV illness.
But it can't help cure or treat children already suffering from serious RSV, and it can't prevent infection.
"The best way to prevent spread of the infection is to always practise good respiratory hygiene, especially people with symptoms that might be due to RSV," Huang said.
One victim is still in hospital after shooting in suburban Auckland.