ANALYSIS:
There will almost inevitably be transmission in schools when Years 1 to 8 students return part-time and Years 9 and 10 students full-time on Wednesday next week.
The unknown is how many, and whether it can be kept low enough to keep the health system from drowning.
With schools in charge of how to mitigate the risks, much will depend on the quality of public health advice to the schools, and how it is implemented.
But there are many factors that make it a strong case for schools to reopen.
Children themselves are far less likely to catch the virus than adults, all other things being equal, and if they do, they're also less likely to get very sick or to infect others.
A study into Delta spread in 51 schools and childcare centres in New South Wales found that children rarely infected other children, teachers or staff.
Most who caught Delta had no or only mild symptoms, and less than 2 per cent were hospitalised. Hospital admission for Australian children have tended to be brief and mild, and sometimes for social reasons such as having a sick caregiver.
That doesn't mean that children can't catch it or pass it on.
The chances of that are higher in areas where community transmission is higher, so schools should be especially cautious in areas that have been flagged as possibly having high levels of undetected transmission: Birkdale, Ranui, Manurewa, Mangere, Sunnyvale, Kelston.
The double-dosed vaccination rates in those suburbs range from the low 70s to the high 80s: 85 per cent in Birkdale North, 87 per cent in Birkdale South, 74 per cent in Ranui and Ranui Domain, 81 per cent in Manurewa Central, 72 per cent in Mangere Central, 80 per cent in Sunnyvale East, 82 per cent in Kelston North, and 79 per cent in Kelston South.
"Transmission in schools generally reflects overall community transmission, so because Auckland is actually on the uptick, I expect we will see some cases," says developmental paediatrician Dr Jin Russell.
"Whether the child actually contracted the infection within the school, I think that's going to be a much lower risk."
A higher risk, Russell adds, is if people gather indoors after school.
Another, as shown in the NSW study, is a student catching Delta, taking it home, and infecting an unvaccinated person in their household.
Of the 59 NSW cases in schools that were followed in the study, 34 were students and 25 were staff. They collectively passed Delta on to 106 people, including 69 students and 37 staff.
But there were 181 cases who were their household contacts.
This is what Covid-19 Response Minister Chris Hipkins was referring to yesterday when he said the chances of spread are greater outside the school than inside.
Those chances will be reduced by several factors: cohorting the part-time return of Years 1 to 8, mask-wearing for those in Years 4 and up, teachers having to have at least one dose of the vaccine by the time the doors open, and ventilating classes by having them outside, or opening the doors and windows.
Studies have repeatedly shown the value of wearing masks.
A report from Arizona showed that schools in two of the most populous counties were three and a half times more likely to have outbreaks if they didn't have any masking requirements.
A recent study of counties in the US showed that mask-wearing more than halved the case rates in schools, from 35 to 16 cases per 100,000 children per day.
In the UK, kids went back to school while the virus was rampant in the community, and without masks. This resulted in an explosion of cases, especially among unvaccinated secondary school students.
So another mitigating factor is vaccination coverage.
For 12 to 19-year-olds in New Zealand, it's relatively high: 85 per cent for one dose and 67 per cent for two doses. In Auckland, where coverage is ahead of the rest of the country, these numbers will be even higher.
As for ventilation, open doors and windows mean the virus doesn't linger in the classroom air for long. In that regard, it's a good thing that we're not in the depths of winter.
We also know that many schools in New Zealand are poorly ventilated, but Hipkins has previously said that it's not worth it to invest hundreds of millions of dollars to install Hepa air filters in 35,000 classrooms.
It will be winter eventually, though, and Russell said it would be prudent to provide schools with portable air purifiers with HEPA filters for rooms that are particularly high-risk or hard to ventilate.
It would be more even more helpful if, by then, 5 to 11 year olds still haven't been able to be vaccinated.
Health boss Dr Ashley Bloomfield said recently that he is expecting Pfizer to submit its data for that age group soon, and it could then be rolled out pending approval from Medsafe, a group of science and clinical experts, and Cabinet.
It's still unclear whether special paediatric doses will be needed, but this seems likely, as it could be haphazard and uneven to dilute hundreds of thousands of adult doses around the country.
The rollout for that age group would then depend not only on the dose being approved, but also on arrival of supply - hopefully by winter 2022, but no timeline has been given.
Another way to mitigate school spread that hasn't been flagged is rapid antigen testing, a tool in the toolkit that the Government has been slow to adopt, despite constant and repeated criticism.
In Victoria, children have these tests twice weekly if they live in Covid hotspots, and they're taken every day for children who are close contacts, instead of 14 days' isolation.
This reduces the loss of face-to-face learning because they can stay in school each day they test negative.
The Doherty Institute in Australia has modelled this approach, and found it had a similar effect on infection rates as a one-week quarantine period for contacts - but students lost fewer days in the classroom.
Testing, contact-tracing and case management, then, have to remain key parts of managing the risk in schools.
The other factor in this complex equation of risk is requiring all teachers to have one dose by November 15, and two doses by January 1. Those who are less than fully vaccinated by the end of the year will need to have weekly testing.
Hipkins announced yesterday that teachers who refuse to have the Pfizer jab will be offered AstraZeneca, but it wouldn't be available until later in the month.
What to do in the meantime about teachers who wanted to wait for AstraZeneca was still being worked through, he said.
Overall, Russell points to the socioemotional impacts of prolonged school closures for children, how risks can be mitigated, and how schools are an "essential service" for children.
She sympathised with parents who are nervous about sending their unvaccinated children back to school.
"There's no such thing as zero risk but these measures will keep the risk low. For most children, Covid-19 poses a similar risk to other seasonal viruses, but we need to work hard to keep cases low, to protect children with pre-existing conditions, and to prevent the sorts of inequitable health outcomes we see for other respiratory illnesses, particularly for Māori and Pacific children."
If the ways to dampen risk are all followed, she added: "Schools are an important part of children's lives and reopening cautiously with prevention measures in place is the right step."
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