COVID-19 Modelling Aotearoa has released details of the modelling it has done to try to predict what will happen with Omicron this winter.
Cases in the first wave were much more concentrated in younger aged groups.
Modeller Michael Plank said that meant older people, as a group, did not have as much hybrid immunity from a combination of being vaccinated and having had the virus.
They also had a higher risk of getting severely ill if they did contract the virus in the second wave, he said.
That meant even if case numbers were the same as the first wave, hospitalisations might not follow suit.
"The number of people getting ill enough to need to go to hospital could potentially be higher if we see increased numbers of infections in those older age groups," he said.
Older people are expected to be on the list of people who will be eligible for a fourth dose of the vaccine later this month.
"Offering a fourth dose of the vaccine as we go into winter for those groups is a really good way of mitigating the risk," he said.
Modelling Omicron longer term was challenging because there were still a lot of questions about how long people had immunity after infection, and how that impacted the spread of the virus through the population, Plank said.
"It's likely that people will become susceptible to being reinfected after time goes by but we don't really know yet what effect that will have on severity. Will we continue to see the rates of hospitalisation we're seeing now or will that severity be blunted over time?" he said.
Another unknown was variants that could arrive and how they could impact on rates and severity of infections.
Aspects of the modelling released last month showed there could be more than 1000 cases in hospital on any given day at the height of the outbreak.
RNZ
