The extent to which children are able to transmit coronavirus, and the difference between the vulnerabilities of younger children and teenagers to the disease, has been a subject of scientific interest as the pandemic has unfolded.
A South Korean study published in a Centers for Disease Control and Prevention journal in July found Covid infection rates among household contacts to be highest where they resided with someone with the virus in the 10- to 19-year-old age group. By contrast, they were lowest in the 0-10 age group.
Dr Simon Clarke, associate professor of cellular microbiology at the University of Reading, said of the study: “It’s the best we’ve got. Children get the virus at all sorts of ages. Because people haven’t been at school, there’s no real epidemiological evidence for whether it’s a problem or not.”
Another study, published in June, found “the added return of most [primarily older] students in Germany has increased transmission among students, but not staff. It is unclear whether older students transmit more, or if physical distancing is practically unfeasible in classrooms at high capacity.”
However, the same phenomenon was not observed in Denmark and Norway, countries with lower community transmission than Germany, the study found. The picture is further complicated by research that found that children under five with mild to moderate Covid-19 have a much higher viral load.
A study from the American Academy of Pediatrics and the Children’s Hospital Association showed that 97,000 US children under the age of 19 tested positive for the coronavirus in the last two weeks of July, which amounts to more than a quarter of the total number of children diagnosed nationwide since March, and a significant rise as some states attempt to reopen in-person education.
Paul Hunter, professor in medicine at the University of East Anglia, said: “My best guess is that an infected young child is more infectious but teenagers have many more close contacts and so are more likely to spread the infection.
R, or the 'effective reproduction number', is a way of rating a disease’s ability to spread. It’s the average number of people on to whom one infected person will pass the virus. For an R of anything above 1, an epidemic will grow exponentially. Anything below 1 and an outbreak will fizzle out – eventually.
At the start of the coronavirus pandemic, the estimated R for coronavirus was between 2 and 3 – higher than the value for seasonal flu, but lower than for measles. That means each person would pass it on to between two and three people on average, before either recovering or dying, and each of those people would pass it on to a further two to three others, causing the total number of cases to snowball over time.
The reproduction number is not fixed, though. It depends on the biology of the virus; people's behaviour, such as social distancing; and a population’s immunity. A country may see regional variations in its R number, depending on local factors like population density and transport patterns.
Hannah Devlin Science correspondent
“If you look at Sweden, that country kept its primary schools open but closed its secondary schools and higher education institutions. So maybe that was enough to suppress the R value a bit.”
Several experts suggest greater social contact might explain older children being more likely to spread the virus than their younger counterparts. However, Dr Sarah Lewis, senior lecturer in genetic epidemiology at the University of Bristol, said: “This could also be due to different immune mechanisms among younger children.”