Nine people across the country have died after becoming infected with RSV this winter - including six in Auckland's central suburbs.
ESR public health physician Dr Sarah Jefferies said the prevalence of RSV was likely the result of lower population immunity because of New Zealand's Covid-19 isolation last year and the increase in movement since then.
She said the "much lower levels of population immunity against RSV" was explained by the Covid-19 response which then led to "exceptionally high RSV activity" in the past five weeks.
Jeffries said the rate of RSV was in decline but it could be because recent school holidays interrupted transmission among children. She warned "as children return to school it is possible that we could see another increase".
The NZ Herald had approached the country's six biggest hospitals to find the impact of respiratory syncytial virus since April.
Auckland District Health Board confirmed last night that six patients who had RSV had died. A spokeswoman said that RSV might not be the cause of the death.
A Canterbury District Health Board spokesman said one patient had died with an "RSV-related complication" and "pre-existing conditions" during this winter's outbreak. He wouldn't specify the patient's age.
Capital and Coast DHB also reported one adult death related to RSV but said none of the children admitted to its intensive care units had died, as of yesterday afternoon.
And the Herald previously reported a the death of a 63-year-old north Auckland woman who had been infected with the virus. She died at her Stanmore Bay home after being discharged from North Shore Hospital last month.
Director general of health Ashley Bloomfield last month said RSV was a "classic" illness New Zealand saw most winters.
"We had very little RSV last year, however this year we are seeing the usual increase that we had seen previously and there's some speculation that it may partly be exacerbated because we didn't have any last year so there is a bigger pool."
DHBs across the country continued to combat a high demand of people presenting with the highly contagious virus, but the latest Environmental Science and Research (ESR) figures showed hospitalisation rates were dropping.
At six of New Zealand's main hospitals, the number of people admitted for RSV spiked to 951 in the week ending Monday, July 18. However, in the week ending Monday, July 25, that number dropped to 787.
As of July 25, there had been a total 4434 hospitalisations for RSV at those six main hospitals.
Southland DHB chief medical officer Dr Nigel Millar said RSV appeared to have eased in southern hospitals, with significantly smaller numbers of children being hospitalised with the virus and other respiratory illnesses.
"From late June, through most of July, children's Wards at Southland hospitals and Children's Unit at Dunedin Hospital were extremely busy caring for children with RSV and other illnesses.
"By the third week in July, the situation in Southland had started to ease but occupancy in Dunedin rose slightly, " Millar said.
The Herald requested the number of deaths those hospitals had recorded and here was the response:
Auckland - 6
Counties Manukau - 0
Waitemata - 0 at the DHB. 1 63yo after the patient was discharged from hospital.
Waikato - 0
Capital and Coast & Hutt Valley - 1 adult
Canterbury - 1 adult
Southland - 0
The Health Quality & Safety Commission, an independent body that reviews adverse hospital events, did not hold "any readily available data" on deaths linked to RSV, a spokeswoman said.
She said specialist advisors for the Child Youth and Mortality Review Committee (CYMRC) and the Perinatal and Maternal Mortality Review Committee (PMMRC) also had no information on RSV deaths. These committees collectively report on deaths of babies and children and up to 24-year-olds.
ESR was continuing to rreceive reports of fresh outbreaks, most recently at three early childhood centres and five aged residential-care centres.
Of cases of severe acute respiratory infection (Sari) monitored at four "sentinel" hospitals in Auckland, around a third involved RSV.
Last year's lockdown cut transmission and squashed rates by 98 per cent as also happened with influenza and other seasonal illnesses.
But it remained less clear how this year's ordeal arose.
Because Australia had earlier recorded RSV outbreaks – and New Zealand's explosion in cases coincided with the opening of quarantine-free travel – scientists strongly suspected the virus had been re-seeded from across the Tasman.
What is RSV?
• Respiratory syncytial virus (RSV) is a highly contagious virus causing lung and respiratory tract infections.
• A person infected with RSV spreads the virus in droplets when they cough, sneeze or talk.
• Very young children and premature babies can become very sick and may require hospitalisation. In this age group RSV can cause bronchiolitis (inflammation of the small breathing tubes of the lung) and pneumonia (infection of the lung).
• Most adults and older children with RSV will experience symptoms similar to the common cold. But older adults, people with compromised immune systems and those with heart or lungs conditions are also more at risk from RSV.
• Many children will be able to recover from this illness at home, but some will be admitted to hospital to support their feeding and to be given additional oxygen when that is needed.