The Government has just extended the vaccine roll-out to Kiwis as young as 12, following provisional approval by MedSafe. What do we know about the Delta variant and young people? And should we be vaccinating everyone? Science reporter Jamie Morton explains.
Is Delta infecting more young people?
The rate of children contracting Covid-19 overseas is still low - but climbing.
In the US, where Delta has become a major variant, children represented 18 per cent of the 674,990 cases reported in the week to August 12 - up five per cent on the fortnight before.
England was seeing a similar increase.
Over the past three months, the proportions of cases aged between two and school year 6, and between year 7 and 11, had climbed from one to 2.1 per cent and one to 3.1 per cent respectively.
Compared with older people and others at higher risk, however, the danger remained low.
As at this month, children accounted for just 0.2 to 1.9 per cent of Covid-19 hospital cases, and only 0.25 per cent of deaths from the virus.
Still, the trend, which has also been seen closer to home in Australia, has worried experts.
Clinicians have aired particular concerns about kids with Covid-19 developing serious conditions, such as multisystem inflammatory syndrome in children, or MIS-C.
This could lead to inflammation around the heart and organs - and without parents even knowing their children had been infected with Covid-19.
The US Centers for Disease Control and Prevention (CDC) was now investigating whether Delta meant more severe disease in children - something that rising cases and restrictions easing made difficult to tease apart.
To Otago University virologist Dr Jemma Geoghegan, the reason for the rise also wasn't clear.
"It's hard to say whether it's because of a lack of vaccine coverage among young people, or that there's been a change to the virus itself," she said.
"Right now, I'm not confident of saying it's either."
Ever since the start of the pandemic, the role of children - and Covid-19's impact upon them - has been debated.
"Early on, it seemed that not only did they rarely become seriously ill, but they did not appear to transmit much infection either," University of Auckland vaccinologist Dr Helen Petousis-Harris said.
"Because of this, they have been largely neglected in the pandemic narrative.
"It does appear that the emergence of Delta has changed this somewhat and we also now know that children are quite efficient transmitters of the virus."
So should we be vaccinating all children?
The Government's move to widen the vaccine roll-out to include children aged between 12 and 15 has been widely welcomed, given the good amount of evidence now available to support it.
Otago University's Associate Professor Tony Walls said that, until there was enough safety data for younger children, he wouldn't be recommending lowering the age threshold.
"For New Zealand, the priority should be vaccinating the elderly and those at high risk of severe complications of Covid."
But if the vaccine was found to be safe for young children, experts say there is reason to use it.
"There is concerning evidence coming through now about the impacts of Covid-19 infection on children, for example potential effects on the developing brain," Otago University epidemiologist Dr Amanda Kvalsvig said.
"We shouldn't be exposing our children to risk from this new virus while these effects are not fully understood."
She added that assessing the impacts of the vaccine was more straightforward because vaccination is studied in a carefully controlled way.
"Here in Aotearoa, we've had an opportunity to assess the results of vaccine trials in children aged 12 to 15 in the US and elsewhere, followed by population-wide uptake in many countries."
In the US alone, just under seven million children in this age group have had at least one vaccine dose, along with a further four million 16 to 17-year-olds.
"With that amount of experience to draw on, the evidence for this age group is very clear: getting vaccinated is a much safer choice than getting ill with the infection," Kvalsvig said.
She said the good results in teenagers raised the question of vaccinating under-12s.
"If the vaccine is judged to be important for protecting younger children, we will need to start making practical arrangements for implementing that decision right away," Kvalsvig said.
"As we're all very well aware by now the Delta variant is extremely transmissible, including in children, and an outbreak can happen at any time."
Being able to vaccinate young children could happen sooner than we might think.
Petousis-Harris said trial results for 5 to 11-year-olds were due at the end of September, and those for 6-month to 4-year-olds shortly after.
"This means that the vaccine could be used in younger children before the end of the year," she said.
"As children are clearly part of the chain of transmission and serious cases can still arise, it is encouraging that they may soon be able to be vaccinated too."
What difference could it make?
Even if we vaccinated children and achieved herd immunity, how long would that last?
"If variants capable of evading existing immunity arise, then obviously we will not be as well protected," Petousis-Harris explained.
"However, that is more about a changing pathogen rather than our immunity.
"Second, if our antibodies decline enough, that may result in growing susceptibility. Thirdly, the quality of our immune memory, and fourthly, the vaccine schedule.
"While Israel are seeing more breakthrough cases than they did earlier in the pandemic, the Delta variant has also been circulating - so one might expect this."
In the US, as of early August, there had been 7101 hospitalisations among 164 million fully vaccinated people.
"In other words, 99.5 per cent of hospitalisations have been in unvaccinated people - so it's holding well."
In New Zealand, modelling has also made it clear that the more people we vaccinate – including children - the lower the risk to the general public.
One such exercise estimated the effect of New Zealand's vaccine roll-out on the spread of Covid-19.
It also explored how many people would need to be vaccinated before we could relax border restrictions and avoid large community outbreaks without the need for lockdowns.
"Our modelling showed that the more people are vaccinated, the less we will need to rely on other measures like lockdowns," Te Pūnaha Matatini modeller Dr Rachelle Binny said.
"Every additional person who gets vaccinated will reduce the number of pathways the virus can use to spread through our communities.
"This will help us to contain the current outbreak and future outbreaks."