"Case rates in the over-70s are already more than double what they were at the peak of the March wave, so that really shows how much the age distribution has shifted," said Professor Michael Plank, of Covid-19 Modelling Aotearoa.
The proportion of infected people older than 60 has also jumped from less than 10 per cent in March, to 25 per cent now.
"We're still in the relatively early stages of this wave – and to be double that level that we were in March is a cause for concern."
Statistically, people aged over 70 are known to be about six times more at risk of hospitalisation with Covid-19 than younger adults.
Ministry of Health data showed that people in their 20s and 30s each accounted for about 17 per cent of Covid-19 cases to date since the start of the pandemic, compared with about 6 per cent and 5 per cent respectively for people in their 60s and those over 70.
Yet, people aged over 60 have made up nearly half of hospitalisations – and about 40 per cent of Covid-19 cases requiring intensive care.
Of 720 deaths where Covid-19 has been the underlying cause, people younger than 60 accounted for 41, compared with 617 among over-70s.
As a "ballpark figure", Plank said hospitalisations – already at the highest levels since early April - could be 50 per cent higher in this wave.
"Age will be the primary driver, but the waning of immunity will be another factor," he said.
"We are modelling the effect of the fourth dose, but we are only just starting to see that come through in the data because people only became eligible very recently.
"Strong uptake will be important for restoring people's immunity back to the level of their booster, and that will help to bring that hospitalisation rate down."
Prime Minister Jacinda Ardern said the increased proportion of older cases was one reason that a shift to the red traffic light setting – focused on stricter gathering limits – wouldn't make a marked difference.
"We really need to make sure they have their booster to make a difference to our health system," she told RNZ.
As at this week, just 91,000 people had received their second booster, which was available to everyone older than 50, along with groups like severely immunocompromised people and healthcare workers aged over 30.
Just why there'd been a clear shift in case rates to older people over time was down to a number of factors, Plank said.
One was that much of the younger population were infected early in the Omicron outbreak and will have gained hybrid immunity that's helped prevent reinfection since.
"Another part of it might be behavioural change, in the sense that, back in March, older age groups were probably being a bit more cautious than your average 20-year-old – and that behaviour might have eased over time," Plank said.
"There's only so long you can avoid crowds or indoor situations and people in their 60s and 70s may have gradually got back to normal."
Otago University epidemiologist Professor Michael Baker said it would be useful for the Government to publish more data that would give older Kiwis a better sense of absolute risk of infection.
"The Ministry of Health has a vast amount of very rich data that needs to be presented in a way that's actually telling people in different age strata about the level of risk – it's not enough to simply say the median age is moving up," he said.
"If you're sitting in Auckland or Wellington or Southland, it's important for you to know just what the risk is for older populations in your area."
This information was important, Baker noted, as there was clear evidence the risk of death and hospitalisation with the virus rose exponentially with age.
"If the virus is circulating much more vigorously, then everyone is at higher risk of infection: it's just the consequences are very different for older people because their immune systems generally don't work as well as they do in younger people – something termed immune senescence," he said.
Baker said that such information could help older New Zealanders know when to take more precautions, notably mask use and greater care with social events.
"It also means we have to do everything we can to stop this virus getting into aged care facilities."
That danger has been highlighted by multiple rest home outbreaks in New Zealand throughout the pandemic, but also across the Tasman.
By mid-May, about one in four of all Covid-19 deaths in Australia had been reported by aged care providers.
Ryman Healthcare, New Zealand's largest retirement village operator, has been managing the threat with infection control plans and requiring staff to be vaccinated and taking rapid antigen tests before work.
Visitors – generally limited to two people at a time for 30 minutes only - were also asked to use masks and complete detailed check-ins to determine any risk and possible symptoms, a spokesperson told the Herald.
Age Concern New Zealand's health promotion and policy manager Joanne Reid also acknowledged it was "concerning we are seeing more older people needing hospital care and that they are more likely to die from Covid-19".
"The best line of defence for older people is to get the second booster as soon as they are eligible, mask up and keep distancing," Reid said.
"We also suggest getting the flu and shingles vaccines to best protect yourself during this winter season."
