Children may be silent super spreaders of Covid-19 because they have high viral loads, a study has claimed.
Youngsters were shown to carry large amounts of the coronavirus in their nose, which scientists say may suggest they have an increased ability to transmit the virus to others.
Infected children with mild symptoms even had bigger viral loads than adults who had been hospitalised by the life-threatening disease.
Having a high viral load — the number of particles of the virus someone is infected with — may make people more contagious, evidence suggests. It can also give the bug a ‘jump start’.
Yet children carrying the virus often show show none of the tell-tale signs, meaning they don’t get tested and unknowingly pose a risk to family members, including their vulnerable grandparents.
The findings came from comparing nose, throat and blood samples of 49 Covid-19 cases under the age of 22 against ones from adults hospitalised by the disease.
But the results are in direct contrast to other studies which have shown children are far less contagious than adults, although not less susceptible to catching the illness in the first place.
Experts said the findings were ‘unfounded’ and ‘misleading’ because the children still had symptoms and would not be walking around schools in real-life.
The role of children in the spread of the coronavirus is highly contested because it has huge implications for the re-opening of schools, which has been the subject of fierce debate in Britain.
Researchers at Massachusetts General Hospital behind the study said: ‘If schools were to re-open fully without necessary precautions, it is likely children will play a larger role in this pandemic.’
Lead author Dr Lael Yonker said: ‘I was surprised by the high levels of virus we found in children of all ages, especially in the first two days of infection. I was not expecting the viral load to be so high.’
In the study, published in the Journal of Pediatrics today, 192 at-risk youngsters up to the age of 22 were tested for Covid-19. More than a quarter (49) tested positive for SARS-CoV-2. But only half had any of the virus’ classic symptoms.
Yet infected youngsters were found to have to have a significantly higher level of virus in their airways than 162 adults who had been hospitalised with the virus.
Viral loads in children reached 6.2 out of ten within the first 48 hours of showing any symptoms, data showed.
In comparison, the figure for adults was just five on average. Dr Yonker said: ‘You think of a hospital, and of all of the precautions taken to treat severely ill adults.
‘But the viral loads of these hospitalised patients are significantly lower than a “healthy child” who is walking around with a high SARS-CoV-2 viral load.’
The ability to spread the virus rises in line with the amount of virus the individual is carrying in their nose and airways, the researchers said, but they did not prove this in their study.
They pointed out children with Covid-19 are not as likely to become seriously ill as adults, which echoes previous studies finding severe disease to be unlikely in young people.
And even when children exhibit symptoms typical of Covid-19, they often overlap with common childhood illnesses, including influenza and the common cold.
The researchers warn that, because children are so often asymptomatic — meaning they show no symptoms, they could be spread the infection at school and bring it home without knowing.
This is a particular concern for families in certain socio-economic groups and multi-generational households where children live with both their parents and grandparents, the researchers said.
In the study, half of children with Covid-19 came from low-income communities — compared to two per cent from high-income.
Along with the viral load, researchers examined how susceptible children were to catching the virus based on expression of their receptors.
SARS-CoV-2 is thought to enter cells by using ACE-2 receptors on cell surfaces, which acts as a ‘doorway’.
Children under ten years old were found to have lower numbers of the receptor in their airways, compared to older peers and adults.
But this did not correlate with a decreased viral load, which the researchers said suggests children can be more contagious, regardless of their susceptibility to developing Covid-19 in the first place.
Professor Alessio Fasano, who was involved in the study, said: ‘Kids are not immune from this infection, and their symptoms don’t correlate with exposure and infection.
‘During this Covid-19 pandemic, we have mainly screened symptomatic subjects, so we have reached the erroneous conclusion the vast majority of people infected are adults.
‘However, our results show kids are not protected against this virus. We should not discount children as potential spreaders for this virus.’
Professor Fasano fears a hurried return to school without proper planning could result in an uptick in cases of Covid-19 infections.
The study recommended routine screening of children for infection and control measures in the classroom, like mask wearing.
But scientists who were not involved with the research said to suggest children are super spreading people is ‘unfounded’.
Dr Andrew Preston, a reader in microbial pathogenesis at the University of Bath, said: ‘In my opinion many of the statements in this paper are largely unfounded.
‘[The conclusions are] very misleading because the study was limited to symptomatic children. It is recognised that symptomatic individuals likely pose a greater risk of transmission than asymptomatic individuals.
‘The discussion appears to suggest children in general will be walking around with high viral loads, when in fact this study was limited to small numbers of children with symptoms.
‘Those displaying symptoms would be isolating, not walking around in schools.’
Dr Simon Clarke, an associate professor in cellular microbiology at the University of Reading, said: ‘In my opinion the headline of the press release is very misleading, because the study does not actually demonstrate that children spread the virus.”
‘It does not demonstrate, in any way, that children actually spread of the virus to adults or other children.”
Adilia Warris, a professor of paediatric infectious diseases at the University of Exeter, said: ‘The study was not designed to assess risk of transmission. Although a high viral load contributes to the level of contagiousness, it is not the only factor playing a role.’
The study also looked at the immune systems of 18 children who developed MIS-C – a recently named condition linked to Covid-19 that scientists have been racing to understand it during the pandemic.
It develops several weeks after infection with Covid-19 and the symptoms are similar, but slightly different, to Kawasaki disease.
The disease can manifest itself as a rash, fever and abdominal pain. But it can be life-threatening, leading to severe cardiac problems, shock and acute heart failure.
The researchers found that children with severe MISC-C were suffering from ‘hyperactive’ immune responses to Covid-19, when the body goes into overdrive and attacks healthy tissue.
It led them to believe that this overreaction by the immune system as it fights the viral disease is behind the mystery illness.
Professor Fasano said: ‘This is a severe complication as a result of the immune response to Covid-19 infection, and the number of these patients is growing.
‘And, as in adults with these very serious systemic complications, the heart seems to be the favourite organ targeted by post-Covid-19 immune response.’