"Vaccinate, vaccinate, vaccinate."
Prime Minister Jacinda Ardern couldn't have been clearer as she set out New Zealand's big pandemic priority to reporters this week.
The country's Delta outbreak has coincided with a surge in administered jabs, reaching the point where two thirds of eligible Kiwis have had at least one shot, and a third are fully vaccinated.
As the year-long drive approaches its most challenging phase – bringing Pfizer to the hard-to-reach and hard-to-sway – there are questions about what our final uptake might look like, and whether it will be high enough to provide more freedom and spare our hospitals disaster.
Much has been said of Australia's 70 to 80 per cent coverage target – a threshold where restrictions across the Tasman would start to be relaxed, albeit with some public health controls staying in place.
Our Government has avoided suggesting such a figure, in favour of pushing the line that it wants as many of us vaccinated as possible.
New Zealand isn't alone: few nations have committed to stated targets, and fewer still have cleared the 70 per cent mark Australia's Scott Morrison has repeatedly touted.
At present, 72 per cent of eligible Kiwis have had at least one jab and 37 per cent are fully vaccinated, compared with the 44.7 per cent of eligible Australians fully vaccinated.
As tempting as it might be to call New Zealand a laggard against other countries' efforts, University of Auckland senior research fellow Dr Janine Paynter said a plethora of hard-to-compare variables were at play, ranging from supply and distribution to logistics and health workforces.
She thought a better yardstick might be our most recent mass-vaccination campaign, launched against Meningococcal B in the mid-2000s - and by that measure we were doing well.
"There were about 2.3 million doses of MeNZB administered in the first year and a bit of the MeNZB campaign," Paynter said.
"This was about 72 per cent of the eligible population vaccinated within the first year."
The eligible population for the Covid-19 roll-out was much larger, but the campaign has managed to notch up more than 4.3 million doses since February, and daily doses in the past few weeks have soared past 75,000.
An extra 750,000 Pfizer vaccines from Spain and Denmark, which come on top of the total 10 million shots New Zealand purchased from the drug giant, should keep stores full as a new vaccination bus service begins venturing into community.
The shot could also soon be administered to children aged 5 and above, pending clinical trial results and approval from regulators, as countries report increasing Delta infections among kids.
How widely do we need to vaccinate to keep Delta at bay?
As an ideal percentage of the eligible population, University of Auckland vaccinologist Associate Professor Helen Petousis-Harris said, "way into the 90s".
"A first dose can keep most people out of hospitals – but if we were to reopen with Delta, hospitals could be overwhelmed."
The outbreak had shown that cases turning up in hospital wards were overwhelmingly unvaccinated – many of them Pasifika people, whom health advocates have argued have been failed by the vaccination programme.
"So, I think we really have to vaccinate virtually everybody who's eligible."
Sobering modelling studies underscore Petousis-Harris' point.
One found that, to achieve population or herd immunity, 98.1 per cent of New Zealand's entire population getting the Pfizer jab – a threshold not far off the 97 per cent Te Pūnaha Matatini researchers earlier arrived at.
Assuming that 10 cases of Covid-19 cases arrived into an open-border New Zealand each day – and that we'd completed a vaccine roll-out targeting high-risk groups, but covering 90 per cent of the entire population – the toll could still be frightful.
Over the two-year period modelled, 1030 people could die from infection and about 11,400 could require hospital care.
It's been estimated influenza already played a role in 500 deaths in New Zealand each year, but Delta's mortality rate among the unvaccinated has been reported to be substantially higher – possibly 10 times more – than most strains of flu.
Delta also spreads faster – it's R0, or basic reproduction number, has been calculated at between five and nine new cases for every one infection, compared with flu's R0 of two – and Covid-19 often comes with more severe illness and long-term problems.
Petousis-Harris added that projections around herd immunity needed to be treated with caution, given that variables like vaccine effectiveness and efficacy, and the virus' rate of spread, made it hard to pin down any magic number.
Immunologist and Malaghan Institute director Professor Graham Le Gros agreed.
"We're much more dispersed geographically and we live in independent housing – so we may find that a vaccination rate of 70 to 80 per cent is more than enough to drive the R0 of this virus down."
Next, he said, scientists had to focus on bringing forward a booster shot, "and developing a system for making long-lived immunity that will last for 10 years".
Fortunately, the Pfizer vaccine was holding up well, even against nastier variants like Delta.
Evidence showed that two doses of the Pfizer vaccine was 88 per cent effective at preventing illness from a Delta infection, and 96 per cent effective at protecting against hospitalisation.
Still, vaccines weren't working as well as they could be to block simple infection and transmission - critical for gaining any population immunity from variants with short incubation periods.
What does that mean longer-term?
"Because the Covid-19 virus is so infectious, and the vaccines are leaky - with moderate but not perfect ability to prevent infection – it's probably not possible to interrupt transmission with vaccines alone," Otago University epidemiologist Professor Michael Baker said.
"Preventing importation of this virus will therefore need to use a combination of vaccination of travellers, information about prevalence of infection in the source country, and some combination of testing and quarantine."
That also meant, he added, that some form of vaccine passport would likely become an important requirement for travel between most countries.
New Zealand was developing one – and was also exploring options for domestic passes that could give Kiwis access to places like clubs and night clubs.
But already, these have been scrapped in the UK – and experts say the concept could worsen inequities here.
That same entrenched inequity was expected to prove yet more of an issue as the vaccine rollout neared the crunch stage.
The most recent survey commissioned by the Ministry of Health, as at June, found that 19 per cent of those not already vaccinated said they were unlikely to have a vaccine – a rate that would proportionately increase as the drive wound on.
Those aged between 24 and 34 years were the least likely to get a vaccine – and the main reasons cited among people unsure or unlikely to receive the jab were feeling they didn't need to, concerns about safety, or waiting to see if others had side-effects.
Among the DHB regions where the likelihood for respondents to accept a vaccine is lower than the national average were Counties Manukau, Northland, Bay of Plenty, Tairāwhiti, Hawke's Bay, West Coast, Whanganui, Nelson/Marlborough and South Canterbury.
But Immunisation Advisory Centre director Dr Nikki Turner said issues among the cohort was much more complex than vaccine "hesitancy" - a term that she felt wasn't helpful.
"Usually, it's a mixture of the system not working very well – like it's been difficult to book, it's not easy to get away to get there, or I've got barriers to accessing the system – and mistrust from health services in the past," she said.
"That means, to actually get high uptake, you've got to look at whether the system is responsive to the customer. And you need communities to be well informed by people they trust and feel confident with."
Among religious groups or migrant communities, local leaders could be powerful agents in helping break down these barriers.
"It's been hard with big numbers, but the system has certainly improved hugely," Turner said.
"Booking systems are working better, clinics are much more accessible for many people now, and primary care and pharmacies are engaged in a way they weren't at the start."
Although vaccinations for Māori were still only sitting around 50 per cent for fully vaccinated people, rates had been rising fast over the past few weeks – particularly in Auckland – and health services were now focused on reaching younger Māori.
The coming months, Turner said, were "where we get 70 per cent of people who've made the effort, get vaccinated and feel good about it. Then we get 20 per cent who need an improved system that makes it easier for them".
"Finally, we get somewhere around 10 per cent of people who really require a lot of extra support.
"You've got isolated people at home, people who've had bad experiences in the past and don't feel very confident, severely needle-phobic people, and then you've just got general mistrust."
According to the ministry survey data, the overall potential uptake rate of eligible Kiwis was sitting around the 80 per cent mark – which wasn't much higher than the 74 per cent rate among Māori respondents.
"My target would be that everybody in New Zealand has been offered a fair and reasonable chance of taking up the vaccine, and that we've addressed both those systems barriers and concerns."
But Turner didn't see merit in an overall target for the country.
"I know a lot of people are calling for it, but what do we achieve by it? If you get 80 per cent, you could do 81 per cent. If you get 90 per cent, you could do 91 per cent. We should aim for the best we can get."
Ardern shared her own concerns around targets this week, telling Newshub that a single milestone for the country might fail to represent regions that had poorer coverage.
"Even if you say 'we want 80 per cent', if you have only 60 per cent in one part of the country, people will die in that part of the country," she said.
"All of us have a role to play in getting our rates as high as possible."
Yet, at the same time, Petousis-Harris said she didn't see the harm in shooting for a figure above 90 per cent.
"We do better than [90 per cent] with our childhood immunisations, right?" she said.
"So, let's not set the bar below where we want it."