Professor Chris Whitty has urged people to get their coronavirus vaccine as soon as possible as he encountered some "very sick" young adults in hospital with covid.
England's chief medical officer said he had spent four weeks working on a Covid ward and told how many "regret delaying" their vaccination.
His comments came as new figures showed that 55% people in hospital with the Delta variant have not been jabbed.
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The data from Public Health England also shows that 74% of people under 50 in hospital with the variant had not been vaccinated.
Almost two thirds of people in the same age group who died in England with the Delta variant were not vaccinated against the virus, the figures show.
Prof Whitty tweeted: "The great majority of adults have been vaccinated.
"Four weeks working on a Covid ward makes stark the reality that the majority of our hospitalised Covid patients are unvaccinated and regret delaying. Some are very sick including young adults.
"Please don't delay your vaccine."
The UK's vaccine programme has so far seen around three-quarters of adults in the UK double-jabbed.
But nearly three million young adults have not had a first dose, according to figures published earlier this week by the four health agencies.
There has been a concerted effort to get as many people vaccinated as possible, with 16 and 17-year-olds getting letters and text reminders this week inviting them for a jab.
The PHE data, published on Friday, showed there were 1,189 deaths up to August 15 of people who were either confirmed or likely to have had the Delta variant and who died within 28 days of a positive test.
While the majority of deaths with the variant were in people aged 50 or over, the under-50s account for more when it comes to hospital admissions.
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