The average length of stay is really low – around two days – and a very small number of these children need oxygen or other help, the data suggests.
Experts say parents are doing the right thing and taking their children to hospital, and there is plenty of paediatric capacity in NHS hospitals to deal with the cases.
The children are being seen by doctors, screened and sent home with good outcomes, they add.
The data was released by the Scientific Advisory Group for Emergencies (Sage) on Friday.
Calum Semple, who advises the Government as part of Sage, professor in child health and outbreak medicine at the University of Liverpool, was part of the team conducting the research.
He said: “Our study has shown that there has been an increased proportion of children being admitted in the last four weeks associated with the onset of Omicron and this has been particularly driven by children under the age of one.
“The striking feature is that we’re now seeing essentially 42 per cent of the children are under the age of one, whereas previously it was around about 30 per cent.
“It does appear to be affecting children from the most deprived areas worse – more than any other group.
“The big question is are these children desperately ill or not? And I really want to emphasise here the fact that these are not particularly sick infants.
“In fact, they’re coming in for short periods of time for investigations.”
The Covid-19 Clinical Information Network (Co-Cin) update on the ISARIC 4C study dated January 12 presents early data which is not yet complete.
The paper says there are proportionally more infants under the age of one, in data from the most recent four weeks from December 14, 2021 to January 12, 2022.
It states “no other strong signal of a change in patient characteristics, severity, or management is seen”.
Russell Viner, professor of child and adolescent health at University College London, said: “I think we’re seeing Covid behaving a bit more like the normal winter viruses that you see in children. So parents are really experiencing that.
“Parents – just do what you always do for winter viruses and children. If you’re worried, talk to your GP – the GPs are open.
“If you’re really worried, ring NHS 111 or take your child to to hospital.
“We hadn’t said this before, but our hospitals are not full, our children’s wards have space, our children’s intensive cares have space, the NHS is open, and is there.”
The Royal College of Paediatrics and Child Health said: “We know that Omicron is much more transmissible than previous Covid-19 variants and results in a higher rate across the community.
“It is therefore to be expected that there will be more children and young people testing positive for Covid-19.
“It is important to note that paediatricians are not reporting Omicron to be a more serious or severe disease in children and young people in the UK.
“We are also reassured to hear that very few children and young people admitted to hospital with Omicron are needing paediatric intensive care.”