Experts have welcomed the Government's move to make the Covid-19 booster available to more Kiwis now, arguing the urgent need to get the third shot out now outweighs any immunity benefit of waiting longer.
Covid-19 Response Minister Chris Hipkins today announced the Government would reduce the booster eligibility interval from four months to three since peoples' second dose – meaning more than a million more New Zealanders aged over 18 would be eligible.
As at today, around 1,362,811 people – around a third of the over-12 population and 69 per cent of those who'd been eligible – have been boosted, while around two thirds have received only their second dose.
UK data has indicated the Pfizer vaccine's effectiveness against symptomatic disease could wane to just 10 per cent over around six months – but the booster could restore this protection to between 60 and 75 per cent after two to four weeks.
After two and a half months, the booster's protection against symptomatic Omicron dropped to about 45 to 50 per cent – and then to 25 to 40 per cent after around four months.
Importantly, the booster shot was estimated at 90 per cent effective at keeping people out of hospital, before dropping to around 75 per cent after 10 to 14 weeks.
Data has also suggested the booster offers a high level of effectiveness against hospitalisation with infections of the ascendant BA.2 sub-variant – which may soon replace the type driving New Zealand's current Omicron outbreak.
Otago University epidemiologist Professor Michael Baker has been among those calling for the Government to follow the lead of several Australian states and cut the eligibility window.
"If you think about the three main things the vaccines does: it protects the recipient, it protects the people around them, and it protects the health system," Baker said.
"Not only will this give people those benefits of more protection against what can be an unpleasant illness, it's also going to help slow the peak of this outbreak and flatten the curve."
Baker said many advantages of the booster rollout would be lost if people were only receiving it after the Omicron wave peaked – which could happen some time in March.
The trajectory of this outbreak remained unclear – whether cases were tracking on an exponential growth curve was likely to be confirmed within days – but experts have said New Zealand at least had a head start against Omicron by boosting many of its most vulnerable already.
Baker felt this change in eligibility meant there was still enough time to get the population well boosted before the worst of the wave arrived.
"If we can get the bulk of adults boosted, and have time to get them some full protection from that, it will help us get through this in the best possible shape."
Otago University immunologist Associate Professor James Ussher said he expected there to be little difference in booster immunogenicity between three and four months after the second Pfizer dose, but added there was a lack of data on the whole.
"I think, given the impending spread of Omicron in the community, and the enhanced protection against severe disease and hospitalisation, as well as against infection, there's an advantage to shortening that interval."
University of Auckland vaccinologist Associate Professor Helen Petousis-Harris agreed.
Just as when the Delta outbreak precipitated the decision to shorten the interval time between the first and second doses of the Pfizer vaccine, New Zealand faced a similar call now, she said.
"It's a bit of a balancing act. Ultimately, if you left it longer, you may get a stronger immune response – but you don't get any benefit from the booster if you don't have it at all."
Malaghan Institute medical director and immunologist Professor Graham LeGros added: "This is a very pragmatic move by the Government to try to get as many people safely boosted as possible before Omicron starts spreading through the community."
University of Auckland senior lecturer Dr David Welch said the Government should now focus on getting everyone eligible booster shots – and also vaccinating children – while Omicron cases were still at relatively low levels.
"We have the ability to administer over 80,000 doses for a sustained period and we should aim to achieve at least that," he said.
"This will greatly increase the immunity we have in the population and reduce the impacts of the Omicron outbreak."
Covid-19 modeller Professor Michael Plank urged the eligible to get boosted now.
"It takes a week or two for your immune system to respond to the vaccine dose. In a couple of weeks' time, it's likely we will have much higher infection rates in the community," said Plank, of Te Pūnaha Matatini.
"So the time to get the booster is now. If you are eligible, please make a plan to get your booster and encourage your whānau and friends to do the same."