Data showing how children are largely being spared Covid-19 should give extra comfort to schools and kindergartens reopening tomorrow, an expert says.

But the pattern remains nonetheless puzzling to epidemiologists around the world.

New modelling has shown that, because children don't tend to spread the disease or become severely ill, reopening schools would be unlikely to result in more cases if there were any outbreaks.

Looking at a hypothetical scenario in which schools had stayed open through the lockdown period, Te Pūnaha Matatini researchers suggested that relaxing restrictions for children would have only had a small effect on the country's total number of Covid-19 cases.

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The modelling didn't create a completely safe picture for schools: if reopening meant much more interaction between adults, such as at drop-off and pick-up time, then the risk of outbreak grew more likely.

But among children, the danger of transmission was low. Although one of New Zealand's biggest Covid-19 clusters was named the Marist College cluster, that was because the first case was a teacher – and most cases didn't involve children.

The modelling was partly based on new data reflecting how the disease was very different across age groups.

The probability of being "subclinical", or generally without symptoms, for instance, was 80 per cent for people under 19, compared with just 20 per cent among those aged over 65.

With the exception of some tragic cases of young victims - today's sole new case was a boy aged under 4, and research has shown children with underlying health conditions face heightened risk - the trend was being seen around the world.

Otago University epidemiologist Professor Michael Baker said it was difficult to see that trend in New Zealand's data, given interventions kept case numbers low, and most were linked to overseas travel rather than community transmission.

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"You've really got to look at a country that's had a sustained epidemic - unfortunately, that's now most of the world – and then ask, what's the pattern after the virus is spreading widely?

"And what's been very consistent is that children under 10 have much less illness."

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Baker cited a study from Iceland that randomly tested 12,232 people and found none of the 848 children under 10 tested positive, compared with 1 per cent of people older than 10.

Another study of residents of the Italian town of Vo also found no cases among children under 10, despite 86 per cent of the population having been infected.

In Chinese data from February, used in one recent review, children and adolescents accounted for just 2 per cent of Covid-19-related hospitalisations.

But the review also noted that, as kids were less frequently symptomatic and had fewer severe symptoms, they were less often tested, which could lead to an underestimate of the true numbers infected.

Also, children were less frequently exposed to the main sources of transmission of the virus.

Another report looking at 15 schools in New South Wales found only one primary school student and one high school student who may have contracted the virus from one initial case at their schools.

"It just seems that somehow, childrens' immune systems are better at managing this infection," Professor Michael Baker says of Covid-19. Photo / Mark Mitchell

"It just seems that somehow, children's immune systems are better at managing this infection," Baker said.

"Not only are they not getting sick, they are seemingly unlikely to be detected. I've been reading about this immunity aspect and, although there are quite a few theories to explain it, basically no one knows.

"I think that is very reassuring for schools, that their environments are relatively safe for children."

To Baker, the low risk to children was one of the most fascinating mysteries about the pandemic, although he added it was common for kids and adults to experience infectious diseases differently.

"This should prove very informative. If we can work out what it is about childrens' immune responses that is having an effect here, it might be key to developing vaccines or other strategies."

How to help kids back to school

Kids will have mixed feelings about heading back to school today, an expert says, and teachers should be closely watching how they adapt to life after lockdown.

"Some children will be so excited about the restart of school they won't sleep the night before," clinical psychologist Jacqui Maguire said.

"Others may experience anxiety upon return or grief that lockdown and family time has ended.

"Conversely, some children may return indifferent, as if the past six weeks didn't occur."

She said research showed children needed to be calm to learn, and if students were experiencing anxiety, teachers could help by keeping classrooms quiet.

"It is also important teachers are aware of their own emotional responses as they return to school," she said.

"Emotions are contagious, and while it is understandable some may be anxious, we don't want this imposed on children. It would be helpful for schools to encourage peer support and professional supervision if required.

"Teacher self-care should be actively promoted, and activities like mindfulness could be undertaken during class time to benefit the teacher and students."

Lessons from the Christchurch earthquakes would also advise schools to set realistic expectations, actively building in time for psychological transition rather than expecting an automatic return to routine, she added.

"While we might be anxious to ensure our students don't academically suffer as a result of Covid-19, initial focus on emotional wellbeing will, in the long run, equal a faster return to optimal learning conditions.

"After all, 2020 is not a usual academic year for students or teachers. And when we are faced with the unusual, we have to flex and adapt to move forward well."

Covid19.govt.nz: The Government's official Covid-19 advisory website