Children have been found to be super contagious spreaders of the coronavirus and may pose a much danger risk to adult populations than previously believed.
As schools plan to reopen, understanding the potential role children play in the spread of the disease and the factors that drive severe illness in children is critical, experts say.
The stakes are rising for teaching staff and families after new research that shows children have a significantly higher level of virus in their airways than hospitalised adults in ICUs for Covid-19 treatment.
Researchers from Massachusetts General Hospital (MGH) and Mass General Hospital for Children (MGHfC) in the US, suggest their findings indicate children may play a larger role in the community spread of the virus than previously thought.
In a study of 192 children aged 0-22, 49 children tested positive for coronavirus, and an additional 18 had late-onset, Covid-19-related illness.
Lael Yonker, director of the MGH Cystic Fibrosis Centre, and lead author of the study, 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.
"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."
Viral load refers to the amount of virus in a person's blood.
Even when children exhibit Covid-19 symptoms like fever and cough, they often overlap with common childhood illnesses, including flu and the common cold.
Dr Yonker says this confounds an accurate diagnosis of Covid-19.
As well as viral load, researchers examined expression of the viral receptor and antibody response in healthy children, children with acute Sars-CoV-2 infection and a smaller number of children with Multisystem Inflammatory Syndrome in Children (MIS-C).
Alessio Fasano, director of the mucosal immunology and biology research centre at MGH and senior author of 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 that the vast majority of people infected are adults.
"However, our results show that kids are not protected against this virus. We should not discount children as potential spreaders for this virus."
The researchers say that when schools reopen it would be ineffective to rely on just symptoms or temperature monitoring.
They emphasise infection control measures, including social distancing, universal mask use (when implementable), effective hand-washing protocols and a combination of remote and in-person learning.
They consider routine and continued screening of all students for Sars-CoV-2 infection with timely reporting of the results an imperative part of a safe return-to-school policy.
However, the study only looked at symptomatic children, and did not measure transmission itself and British experts have warned people ought to execise caution.
Adilia Warris, professor of paediatric infectious diseases at the University of Exeter, said: "The authors do show that children who presented with respiratory symptoms during this pandemic, and who tested positive for Sars-CoV-2, displayed viral loads comparable to adult hospitalised patients, especially in the first two days of symptoms.
"Interestingly, of the children presenting with symptoms, only around 28% of children tested positive, and of these, more than 60% were over the age of 11, 26% were obese (with less than 10% in the non Sars-CoV-2 group), and exposure to the virus was by either mum or dad (77%), supporting a larger role for adults in the transmission of this virus.
"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 was performed in children presenting and/or admitted to hospital, which we know is different from children presenting to community practices, and therefore the conclusions and translations the authors make with respect to schools is in my opinion too far-reaching, and is not supported by the data they present."
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