There are pockets in Northland and the East Coast/Tairāwhiti sitting at less than 40 per cent vaccinated, which Ashley Bloomfield says are like fire to a heat-seeking missile for the virus.
The director general of health said a 90 per cent vaccination rate of the eligible population was critical across all groups and all regions of New Zealand.
And it wasn't just the key to borders opening up earlier, but also to a freer life in a Covid-world.
Asked what would happen if 90 per cent wasn't achieved, he brushed it off as an inevitability. "I don't think it's a matter of 'if', it's just a 'when'. Obviously, the sooner the better."
It could also be derailed if a new variant emerged that was resistant to the Pfizer vaccine, and while there was no sign of this, it wasn't impossible.
Prime Minister Jacinda Ardern has identified three key areas where vaccination needs to be high enough - without saying how high was high enough - before a phased reopening of the border could start in the first quarter of next year.
Those are young adults (20 to 34), who spread the virus more because of busier social lives, the vulnerable including 65s and over and Māori and Pasifika, and regions.
"What we can't afford to have is vulnerable communities in places like Northland, Tairāwhiti," Bloomfield said.
"If the virus is in the country, it's like a heat-seeking missile. It'll find those pockets and then you will get a lot of infection, and a lot of people will get very unwell."
He said health providers, particularly Māori providers, were already looking at low coverage spots at a neighbourhood level.
"So they could know the areas, and even the households, that haven't been vaccinated and get out there and go door-to-door. Making it easy for people to get vaccinated is absolutely critical.
"Some groups are down in the 30s and 40s [per cent]."
The lowest rates of vaccination among 12 to 34-year-olds with one dose are in Northland (44 per cent) and Tairāwhiti (48 per cent), nudging up by 1 percentage point each if first dose bookings are included.
Bloomfield said he was "most interested" in the young adult group, particularly Māori and Pasifika.
Māori in the 20 to 34 age group are vaccinated at just over half the rate of non-Māori non-Pacific.
Across all eligible ages, only 55 per cent of Māori nationwide have had a first dose. Nationally, the rate is 78 per cent.
For the estimated 1.7 million vulnerable people in group 3, Health Ministry data shows only 765,000 people with at least one jab - including 728,000 people aged 65 and over.
Bloomfield insists this doesn't mean one million vulnerable people are completely unvaccinated.
"It suggests that we didn't really have a good handle on what the denominator was. Many people got vaccinated and counted in group 4."
It's unclear if this means the ministry has eyes on the vaccination status of those under 65 with underlying health conditions.
Vaccination carrots: Open borders and fewer lockdowns
Bloomfield said high vaccination coverage would mean fewer, but not zero, restrictions.
"Even in jurisdictions like Singapore and a number of European countries - there is a handful like Portugal, Ireland, that have vaccination rates over 90 per cent - they still have restrictions in place."
Ardern has talked about a reluctance to tighten restrictions at level 1 beyond mandatory QR scanning, but Bloomfield had previously floated whether more open borders would need to be balanced by level 1.5 restrictions, including wider mask use (he mistakenly said 2.5 at the time).
Ardern has also talked about restricting high-risk events - but not essential services like supermarkets - to only the fully vaccinated.
For Delta, the vaccine was very good at minimising symptomatic infection, but Bloomfield said evidence was still emerging about how good it was at reducing transmission.
He added there was "no doubt" that having more than 80 per cent of eligible Aucklanders with at least one jab was helping to manage the current outbreak.
Hitting 90 per cent didn't mean lockdowns would no longer be needed.
"You may need them for local outbreaks - and we've shown we can manage boundaries. It's tricky but we can do it.
"It's also possible there will be another future variant that will require us to re-vaccinate the population. There's no hint of it at the moment, but we've got to remain open-minded."
Tapering off at 80 per cent?
Surveys commissioned by the Ministry of Health suggest coverage of the eligible population might start to taper at 70 to 80 per cent, and Ardern has acknowledged getting from there to 90 will be "really tough".
Bloomfield said he was "actually quite confident" of hitting 90.
"In our over 65s we've got well over 90 per cent, and that's across different groups.
"We've also been in a position with all our groups across the country at about 90 per cent with childhood immunisation, so we've got a track record."
Ministry of Health data shows different 12-month child immunisation rates across different age groups, ranging from 92 per cent for 1 year olds to 73 per cent for 4.5 year olds.
Bloomfield added: "It's not just possible, but also the modelling shows it's kind of essential."
With moderate public health measures, Te Pūnaha Matatini modelling shows the difference between 80 per cent (1.3m cases, 58,464 hospitalisations, 7004 deaths) and 90 per cent (461,893 cases, 13,796 hospitalisations, 1557 deaths).
If the eligible population included children aged 5 to 11 - which Medsafe is considering - the estimated number of deaths drops to 4936 at 80 per cent, and to 50 deaths at 90 per cent.
Protecting children who can't get the vaccine was a "big motivation", Bloomfield said, as were the health services that wouldn't be available if wards were filled with Covid patients.
"Every time an ICU bed is used for a Covid patient, it can't be used for someone who might be missing out on their heart surgery.
"We need the system to be able to deliver all care that other people need, and a high vaccination rate just makes it much more possible."
2022: Effective treatments, antigen tests at home, tracking bracelets instead of MIQ
Public health measures next year would likely include at-home rapid antigen testing, which the National and Act parties argue should already be in place.
Such tests are less reliable than saliva or nasopharyngeal methods, but results can be returned within 15 minutes.
They were being piloted at the ED in Middlemore Hospital, and could be used as part of the self-isolation pilot for 150 business travellers starting later this month.
"It almost certainly will be used more widely," Bloomfield said.
"Where it's useful is where we've got a group that might be potentially exposed because they're in public-facing roles and you can do it regularly, for example on a daily basis."
But care in their use had to be taken, he added.
"Germany is pulling back on doing rapid antigen testing because the sense is it's stopped some people from being vaccinated. People are using this as an alternative."
He also said smart bands or bracelets that could track a person's location and let the user know if they've been exposed "may be useful".
"It's part of a suite of measures you could use to replace the use of managed isolation."
This follows criticism over the Health Ministry's limited use of the Bluetooth function in the Covid Tracer App in dealing with the current outbreak.
The World Health Organisation has also recently supported the Ronapreve monoclonal antibody cocktail.
A study - yet to be peer-reviewed - showed it could reduce the chances of hospitalisation and death by 70 per cent, and shorten the duration of symptoms by four days.
Infectious diseases physician Professor Kurt Krause said the treatment had showed "strong evidence of effectiveness at preventing symptomatic infection following exposure".
"But they must be given early in the course, preferably within 96 hours of the index case having a positive test," he added.
Bloomfield said Ronapreve looked "promising" but was "quite expensive". It was being considered by Medsafe.
But he didn't want people to use the treatment as a reason not get vaccinated.
"Treatments will be helpful, and there will be progress on treatments quite quickly. We've seen it already, but vaccination is the number one tool in the toolbox."