One in five people being admitted to hospital in England with Covid is aged between 18 and 34, the new chief executive of NHS England has said.
In her first major interview, Amanda Pritchard told the BBC that about 1,000 young adults were currently "really unwell" in hospital.
She said it was "so important" people came forward to get vaccinated.
She added the level of young adults being admitted to hospital was four times higher than the peak last winter.
About 5.5% of those in hospital during the winter surge were young adults.
NHS sources said the latest figures on hospital admissions reflected the success of the vaccination programme in protecting older age groups.
Ms Pritchard said: "It shows how effective the vaccine programme has been in protecting people, stopping them needing hospitalisation, keeping them safe."
She warned that young people who have not been jabbed could become seriously ill, adding, they "are not immune and the best way they can protect themselves absolutely is to get that vaccine if they haven't already".
Last week, about 250,000 18-30 year-olds had their first or second dose of a Covid vaccine, which was "incredibly encouraging", said Ms Pritchard.
"It is still really important for those young people who have not yet taken the opportunity to come forward - or they know someone who hasn't - this is the time," she told the BBC's health editor Hugh Pym.
In one sense the latest figures showing a sharply higher proportion of under 35s among Covid hospital patients illustrates the success of the vaccination programme.
Back in January, when it was early days for administering the jabs, older people who were more vulnerable were more likely to get seriously ill with Covid and need hospital treatment. Now almost all have the protection of two doses.
Total hospital numbers in the latest wave have not been close to where they were during the winter and they are now levelling off.
But they rose rapidly in July and now we have one indication of the extent to which that was down to younger unvaccinated patients getting sick with the virus.
It serves as a reminder that people in their late teens and twenties who might not have been jabbed can get ill enough to need hospital treatment.
She said although hospital admissions have flattened out over the last few days and there were grounds for "cautious optimism", the NHS was still experiencing "real pressure".
Ms Pritchard, who was previously NHS England's chief operating officer, took over as chief executive from Sir Simon Stevens on 1 August.
In a statement issued by NHS England, she added: "NHS teams are putting on pop-up clinics and walk-in centres in addition to around 1,600 permanent sites, to make it as easy as possible to protect yourself, your family and your friends."
Last month, there were suggestions that vaccines could become compulsory for university students moving to halls of residence, but ministers later abandoned the idea.
About 73% of adults in the UK have now received two doses of a Covid vaccination, and nearly 89% have had a first dose.
On Wednesday, ministers in the UK's nations all said they would offer Covid jabs to all 16 and 17 year olds following advice from experts on the Joint Committee on Vaccine and Immunisation.
It comes as a further 29,312 cases of people testing positive for coronavirus were recorded in the UK on Wednesday and 119 deaths within 28 days of a positive test.
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