Around "75 to 100 children" in the UK have been affected by of a rare syndrome linked to coronavirus, the president of the Royal College of Paediatrics and Child Health has warned.
Evelina Children's Hospital in London saw a cluster of eight cases over a 10-day period in April, and had seen around 50 children overall with the illness.
Great Ormond Street Hospital has also treated patients with the syndrome, which has been compared to Kawasaki disease.
One patient, a 14-year-old boy with no underlying health conditions, died after spending six days in intensive care at Evelina Children's Hospital.
According to a report by his medical team published in The Lancet journal, his main symptoms on being admitted to the hospital were a temperature over 40°C, diarrhoea, abdominal pain and headache.
Speaking to BBC Radio 4's Today programme, Professor Russell Viner said the "new syndrome [...] appears to be happening mostly after coronavirus infection, we believe it’s where the body’s immune system overreacts to coronavirus."
He added there were "very few cases, 75 to 100 across the country", and stressed: "The important thing to say is most are being treated well, many are going home, most haven’t gone to intensive care units."
Dr Sanjay Patel from Royal College of Paediatrics and Child Health told ITV News "around 50 children" had presented with symptoms that required critical care.
Dr Sanjay Patel tells ITV News about the symptoms of the rare syndrome:
Dr Patel advised parents that the risk of children contracting the syndrome was "extremely low".
The condition is said to be similar to Kawasaki disease, which mainly affects children under the age of five, with symptoms including a high temperature, rashes, swelling and a toxic shock style response.
Last month, the UK Paediatric Intensive Care Society (PICS) tweeted an alert that warned of a "rise in the number of children [...] presenting with a multi-system inflammatory state".
At the time Health Secretary for England Matt Hancock said experts were investigating the syndrome in children "with great urgency", but has stressed it is rare.
Dr Hanna, a consultant in children's intensive care, said those with the illness have a "reasonably long stay in hospital", with some admitted for up to two or three weeks.
She told the PA news agency: "The majority are in high dependency, so not the highest level of intensive care.
"We have got a handful of children in intensive care but they are not currently requiring the highest levels of support, which is good as some of the children we have had prior to this have been much sicker."
She said all of the other cases had survived, apart from the 14-year-old.
Most of those with the illness are of school age - while there have been a small number under the age of five - and the majority are fit and well with no underlying health problems.
A "small number" tested positive for Covid-19, Dr Hanna said, but this is the minority.
Great Ormond Street Hospital, where cases have also been reported, has worked on antibody testing with the Evelina Hospital - this found evidence that children who are ill with the syndrome have previously had Covid-19.
Dr Hanna said there is "circumstantial evidence" that the two illnesses are related, but added: "I think most people believe that they are, because it is so temporally associated."
Patients are being given general treatment, specifically anti-inflammatory agents - ranging from aspirin to specialist therapies - steroids and immunoglobulin infusions, while some are receiving more advanced drugs.
Dr Hannah said: "It's a very small number of children in terms of the population, but it is a large number of children related to our hospital."
Dr Liz Whittaker, clinical lecturer in paediatric infectious diseases and immunology, at Imperial College London, said the fact that the syndrome is occurring in the middle of a pandemic, suggests it reasonable to think the two are related.
She said: "You've got the Covid-19 peak, and then three or four weeks later we’re seeing a peak in this new phenomenon which makes us think that it’s a post-infectious phenomenon, and that it’s more likely to be something maybe either antibody mediated or something that we call an immune complex."
Professor Russell Viner, president of the Royal College of Paediatrics and Child Health, said the majority of children who have had the condition have been well treated, responded to treatment and are getting better and starting to go home.
Although much of the conversation has been around children having the syndrome - the experts say it cannot yet be ruled out in adults.
Dr Whittaker said: "It may not be specific to children.
"One of the things that the study is going to look at is the adult data to see if perhaps some of the adults who are being admitted, and just being called Covid-19, may actually be presenting like this – particularly some of the younger adults."
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