How children get and transmit COVID-19 is still a mystery

Small child in window of house with facemask COVID-19

Despite the unequivocal assurances provided by the Deputy Chief Medical Officer and the federal Education Minister, much about COVID-19 remains a mystery. There’s still a lot we don’t know about how the virus affects children. That means the stakes are high when deciding whether kids go back to school now, either full or part-time, or remain at home for now and return at the start of Term 3.

Children can get COVID-19, but we don’t know whether they are less likely to become infected

One thing we do know is that children of all ages can get COVID-19. There have been 271 confirmed cases of children with COVID-19 in Australia so far. Some studies, discussed below, suggested they were less likely to catch the virus than adults, but good recent evidence suggests children may be just as vulnerable.

Under symptom-based testing, such as has been the policy in Australia until recently, asymptomatic people with COVID-19 are unlikely to be tested and diagnosed. So symptom-based testing tells us how many symptomatic people tested positive for COVID-19, rather than how many people have COVID-19. Tests are mostly done on people who appear sick, and asymptomatic people, by definition, do not appear sick. Evidence so far suggests many children with COVID-19 are symptom-free.

To discover the COVID-19 rates for children, people without symptoms need to be tested. This happens in two scenarios: when all close contacts of a confirmed case are tested, and when a random sample of people is tested.

In Iceland, 10,800 asymptomatic people were tested for COVID-19 in mid-to-late March. About 0.8 per cent were positive. Of the 848 children under 10, none had the virus; of the 1200 children aged 10-19, 0.4 per cent tested positive – half the rate of the adults.

In Germany, a COVID-19 lab processed 60,000 tests. Of the 2,200 children under 11, 2 per cent tested positive; of the 1,900 people aged 11-20, 4 per cent had the virus. About 22,000 adults aged 20-40 were tested over the same period, suggesting they were more likely to have symptoms. But only 5 per cent of them were positive for COVID-19, not much higher than the rate for the children.

In a well-designed study, the Shenzhen Center for Disease Control and Prevention identified 391 COVID-19 cases and 1,286 close contacts. It looked at people in households with a confirmed COVID-19 case. The authors found that children were ‘just as likely’ to contract the virus as adults under 50.

That finding runs counter to analysis published a few days later, in which researchers examined the contract tracing information from the CDC in Hunan, China. Contacts to COVID-19 positive patients were placed under medical observation for 14 days. Analysis of people’s susceptibility to the virus concluded that children did have a lower risk of infection.

Children can spread COVID-19, but we don’t know whether they spread it to fewer people

Another thing we know is that children can pass on COVID-19 to adults and other children. But observational evidence so far has shown that they are less likely to spread the virus than adults.

At the beginning of the COVID-19 epidemic in Australia, the National Centre for Immunisation Research and Surveillance (NCIRS) studied 9 adult and 9 child cases of COVID-19 in 15 NSW schools. The study identified 832 ‘close contacts’ – 735 of them children. One-third of this group were interviewed, tested with nasal swabs 5-to-10 days after contact, and had a blood sample examined for antibodies one month later.

One child in primary school tested positive on both the nasal swabs and for antibodies; and one child in high school tested positive for antibodies, but not on the initial nasal swab.

These cases happened in early March, before government recommendations for spatial distancing and lockdowns. Back then, Australia had done very little to reduce the spread of COVID-19. That two children out of 288 tested positive indicates that child-to-child transmission is possible, but suggests the rate of transmission is low.

The NSW study is in line with other observational studies. In a case study of an outbreak in the French Alps, a symptomatic child visited and had ‘close interactions’ in three schools without passing on the virus to anyone. The authors said this suggested ‘different transmission dynamics in children’.

A multinational study of 33 household clusters found that a child under 18 was the initiating contact (‘index case’) for three. The authors of this study note that this is well below otherwise similar infections such as the H5N1 influenza virus, in which children are the index case about half the time.

In the Netherlands, the Ministry of Health studied 54 households with COVID-19 infections and found that while children did become infected, they were never the source of the spread.

Whether or not schools were open at the time of infection is important for studies that examine household index cases. A detailed study of 36 paediatric cases in Zhejiang, China, found that almost all children got the disease from family members rather than the community or other children. But schools were closed for the spring festival holiday during this period, so child-to-child contact was drastically reduced.

One thing we don’t yet know is why an infected, symptomatic child would spread the disease less than an infected, symptomatic adult.

The virology data to date suggest children are as infectious as adults. A study of 3,700 COVID-19 patients in Germany found there was no difference in the viral load – a measure of infectivity – between people in different age groups, including children. The virologists concluded that their findings, combined with the evidence of children’s vulnerability to infection,suggested that the ‘transmission potential in schools and kindergartens should be evaluated using the same assumptions of infectivity as for adults’.

Children with COVID-19 are less likely to become severely ill, but we don’t know whether they suffer long-term effects

We know that children with COVID-19 can become severely ill. But the available evidence strongly suggests they become severely ill at lower rates than adults.

In a comprehensive study of 2,135 paediatric cases in China, more than half had mild (flu-like) symptoms at worst. About 40 per cent had moderate symptoms, such as pneumonia, frequent fever, and dry cough. The remaining 5 per cent were classified as severe or critical, compared to 19 per cent of adults in China at the same time.

While the severe-illness rates for children with COVID-19 are low, the medium- and long-term effects are still unknown. This week the UK Health Secretariat warned of a serious emerging syndrome affecting children, potentially related to COVID-19. There have been similar reports in Italy. Only time and regularly updated research will tell us how serious this is.

We do know that children can and do die from COVID-19. There have been deaths of children in China, the United States, the United Kingdom, France, and other countries with substantial outbreaks. Death rates of children with COVID-19 are very low. But there are 4 million school children in Australia, meaning that in an outbreak, even a low death rate could translate into the deaths of many children.

In the face of this uncertain evidence, in deciding whether to open schools, policy makers have to weigh up the evidence – what is the likelihood of infections, of passing that infection along, and what are the potential health and economic consequences. Because we have seen no child deaths from COVID-19 in Australia, decision makers may be inappropriately ignoring that possibility. We all are prone to ‘optimism bias’ – erring on the positive on all the issues which should be taken into account in the difficult decision.

Although children are much less likely to get seriously ill and die, it is possible this will occur. It is probably the case that they are less likely to transmit the virus than adults, but nevertheless they can be the primary source of transmission. It is wrong for decision makers to pretend that the evidence is clear when it is not. Opening schools is not a risk-free choice and should not be portrayed as such.

There is uncertainty around COVID-19 and its effects on and transmission through children. We are safer if we make decisions while fully aware of that uncertainty, rather than with an unfounded surety. We need to know how firm the ground is under the science that guides our decisions. Only then can we properly assess the risks, measure the trade-offs, and make the tough decisions that need to be made about our schools – and about protecting our children.

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