A record number of children in England were admitted to hospital with COVID on 3 January, according to government data.
Some 157 children were admitted on the Bank Holiday Monday, 110 of whom were aged 5 or younger.
The figure surpasses the previous record on 145 admissions on 28 December.
In the last seven days, a total of 567 children have been admitted to hospital with COVID.
Professor Deepti Gurdasani, a senior lecturer and epidemiologist at Queen Mary University of London, wrote on Twitter: "I really can't understand the lack of attention and discussion of this.
"This is huge.
"Shouldn't there be urgent focus on trying to understand what's going on here?"
"570 0-5 year-olds being hospitalised with COVID-19 [in just a week] and rapidly rising isn't ok."
The COVID case rate among young children is the highest recorded so far during the pandemic, with infections rising rapidly among 0-4 year-olds.
Cases have dropped among school-age children over the Christmas holidays, but are expected to rise again as children return to the classroom.
There are currently 17,988 people in hospital with coronavirus in the UK, up 50% in a week.
During the second wave, the number peaked at 39,254 on 18 January 2021.
Cases have been rising rapidly in the UK in recent days, with a record 218,000 infections confirmed on 4 January, and a further 179,000 reported on Thursday.
Watch: PM insists reports of NHS having too few staff 'not true'
The rise in cases and hospitalisations among children comes as a member of the government’s Scientific Advisory Group for Emergencies (Sage) said it was likely the vaccine rollout will be extended to include children as young as five.
Currently only clinically vulnerable five-year-olds can get a vaccine.
Speaking in a personal capacity, he said: “Five to 11s are probably the group least affected by COVID disease..
“The thing about COVID is it’s got the most extraordinary age risk profile… to be honest, five to 11 is the healthiest time of our life. It’s the time when we’re least likely to die or get sick from almost anything, and that is true of COVID.
“However, I expect and I would like [the government] to include educational disruption and mental health issues in the decision, which is what happened with teenagers.
“I think it’s a very marginal medical decision, but if you include those broader issues, I think given the extremely promising safety profile in children – I don’t want to second-guess the JCVI – but I think the balance of risks is towards vaccination.”