A Covid-19 modeller is warning a second Omicron wave, likely in the next three to four months, could result in higher numbers of hospitalisations and deaths.

Covid-19 Modelling Aotearoa has released details of the modelling it has done to try to predict what will happen with Omicron this winter.

"So far most of our cases have been concentrated in younger age groups, but there's a risk that if we start to see increasing infections in those older age groups that will flow on to higher hospitalisation numbers and higher deaths," says modeller Michael Plank.

As a group, did not have as much hybrid immunity from a combination of being vaccinated and having had the virus, as younger age groups.

They also had a higher risk of getting severely ill if they did contract the virus in the second wave.

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," says Michael.

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," says Michael.

"That fourth dose is going to be really important to reduce the risk of a big impact from a second wave for older people and others in high risk groups like immuno-compromised people, and Māori and Pasifika."

Uptake of the third vaccination among younger people had been "patchier" than in older age groups, he said.

"It would certainly be good to see increased uptake in those younger age groups.

"It's really important people understand that just because they've had Covid doesn't mean you're not going to get it again, it doesn't mean you're not going to get it again more seriously second time around, and the evidence increasingly shows that the vaccine gives you stronger immunity."

A second wave was likely some time in the next three to four months, as immunity waned after infection.

"Given the timing of our first peak in March, now is the time when we're potentially going to start seeing increasing numbers of reinfections, because people are going to be in that period when they're more than 90 days since their first positive test."

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, says Michael.

"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?"

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.