Most eligible New Zealanders have yet to get the new Covid-19 booster and an expert says lifting uptake is crucial to reducing the number of people hospitalised or killed by the virus this winter.

The Government rolled out the updated Omicron-targeted booster to Kiwis over 30 at the start of April, to offer people an immunity top-up ahead of winter and help ease pressure on the health system. For those up to date with their vaccinations, this is the fourth or fifth Covid jab they will have received since the start of the pandemic.

Yet, between April 1 and May 12, Ministry of Health figures showed only around 411,000 boosters - nearly all of those second and third ones - had been administered.

While a chunk of New Zealand’s over-30 population of 3.1 million wouldn’t have been eligible if they’d been boosted or infected in the past six months, Herald analysis shows the majority of the pool was yet to take up the free shot.

Ministry data also shows that only half of Kiwis older than 50 have received more than one booster, despite the Government offering second ones to that age group last winter.

As with that roll-out, weekly uptake rates of the bivalent booster have only reached as high as about 100,000.

That was five times less than the height of uptake for the first Covid-19 vaccine - and four times less than peak uptake of the first booster.

For the many Kiwis last boosted more than a year ago, neutralising antibody levels would have dropped away substantially, leaving the door to reinfection open ever more widely over time.

The good news was that infection or vaccination did give us some long-lingering immune memory to work against the virus, at least in any form it’s taken so far.

That protection included T cells - the natural hunter-killers of our immune system that can instantly recognise infected cells - as well as B-cells, which begin pumping out fresh neutralising antibodies once activated.

While this back-up immunity wasn’t enough to stop the virus invading our bodies, in many healthy adults, it did stop it making us sick enough to need hospital care, or die.

Those who’d acquired “hybrid immunity” from being vaccinated and previously infected - as most Kiwis now would have - would be likely to have the highest magnitude and durability of protection.

But experts say boosting remains critical for lifting immunity, especially among older or more vulnerable people.

“It’s much better to top up immunity with a vaccine than infection, so getting good uptake of the new booster is really crucial for reducing the number of hospitalisations and deaths over the coming winter period,” said Professor Michael Plank, of Covid-19 Modelling Aotearoa.

Only a third of people aged 50 to 64 had received their second boost, while uptake rates among over 65s - in which the bulk of hospitalisations and deaths to date were concentrated - remained at just under 70 per cent.

“It’d be really good to see higher rates, particularly in that over-65 group,” Plank said.

While case rates among older Kiwis had been coming down over time, Plank and his colleagues had been watching numbers rise among children since the start of this school term.

Plank added that the bivalent booster - specifically targeted at the BA.5 subvariant that fuelled our last winter wave, and also available to people over 16 at higher risk of infection - had been shown to perform better against Omicron types than the first booster.

One recent US study showed bivalent boosters to be some 37 per cent more effective than older ones at cutting the risk of a severe infection.

University of Auckland vaccinologist Associate Professor Helen Petousis-Harris said that, although each successive boost came with “diminishing returns”, getting the booster was encouraged for many Kiwis.

“If you haven’t been recently infected and it’s been a long time since your first boost, then you’d benefit from getting an additional one,” Petousis-Harris said.

“And if you’re in one of those higher-risk groups, then you should be paying special attention to your booster status.”

That’s a message underscored by the findings of a newly-published study, which analysed Covid-19 cases in the Midland health region between February and June last year.

University of Auckland vaccinologist Associate Professor Helen Petousis-Harris said the booster was encouraged for many Kiwis. Photo / Dan Cook, RNZ

The Waikato University-led study - one of just a few taking a detailed analysis of Omicron’s impact - found the hospitalisation risk for unvaccinated people was more than four times higher than those who’d received three doses.

Again, age mattered: the hospitalisation risk for elderly people with Covid-19 was shown to be nearly 40 times higher than those aged 12 to 24, while the average length of hospital stays was five times longer for over 75s than for children younger than 11.

The research also highlighted a worrying equity gap that’s marred New Zealand’s pandemic response, with the highest - and earliest - per-capita Omicron cases recorded among Pacific and Māori people.

“Given we’d predicted these inequitable outcomes beforehand, it was disappointing that we ended up with them,” lead author Dr Jesse Whitehead said.

“In the future, we definitely need to do a better job of making sure those priority populations are better supported to get vaccinated.”

Te Whatu Ora’s interim prevention direction Astrid Koornneef also encouraged Kiwis to take up the booster - along with other vaccines they were eligible for - ahead of the colder months.

“Taking practical steps to make sure you don’t get seriously unwell is the first line of defence this winter,” Koornneef said.

“This means getting your Covid-19 booster and your flu vaccine, and making sure your tamariki have all the vaccines they need to protect them.”

People can book their Covid-19 booster online through bookmyvaccine.health.nz, or by calling Healthline on 0800 28 29 26.

The latest seven-day rolling average for Covid cases is 1672, with eight virus-related deaths in the week to May 15. At that time, 247 people were in hospital with Covid.