This article was amended after Sir Patrick Vallance corrected an earlier statement in which he initially said 60% of hospitalisations were double vaccinated people
Around 6 in 10 of those requiring hospital treatment are unvaccinated, the government's chief scientific adviser has said.
Speaking at a Downing Street press conference on Monday, Sir Patrick Vallance said it was "inevitable" that the proportion of people being admitted to hospitals who have had both jabs will increase.
It comes as most remaining lockdown measures were eased in England as cases continue to rise.
The government has justified easing measure by arguing that the vaccine program is helping to sever the link between catching the virus and serious illness and death.
About 36 million - or 68% - of all adults have add the second jab, while 48 million - 88% of adults - have had one jab.
But recent figures show that the number of people, who have received both doses and are catching the virus with some then being hospitalised, is rising.
On Monday, according to the latest government figures, 742 people were admitted with COVID. That means the total admitted in the past seven days stands at 4,317 - an increase of 40% on the previous week.
Sir Patrick said: "Vaccines are not 100% effective. They're very very effective, but not 100%
"As a higher proportion of the population is double vaccinated, it's inevitable that those 10% of that very large number remain at risk and will therefore be amongst the people who both catch the infection and end up in hospital."
Sir Patrick also reiterated the vaccine is less effective at stopping people catching and spreading coronavirus that it is at preventing serious illness.
Watch: Those self-isolating must continue to do so - PM
"If everybody over the age of 18 had taken up the vaccine then anybody who caught it would be double-vaccinated," he said.
"We should expect to see a higher proportion of people in hospital and catching the infection that are double-vaccinated because of the less than 100% efficacy oft he vaccines overall. "
Boris Johnson also told the press conference that the number of people catching COVID despite being double jabbed had been rising.
Responding to a question over the proportion of daily cases who had had zero, one or two doses, he said: “I think, sadly, the number of COVID cases that have involved somebody who’s had two vaccinations has been rising, although clearly the results… of the vaccines remain very good in the sense that they protect those people very largely against serious illness and death, even if they contract it.”
It comes after the government revealed that COVID vaccine certifications are likely to be brought in for crowded, enclosed venues such as nightclubs.
Speaking to the House of Commons on Monday, vaccines minister Nadhim Zahawi said proof of a negative COVID-19 test would soon “no longer be sufficient” that a person was COVID-safe.
He urged businesses to “use the NHS COVID pass in the weeks ahead”, adding: “We will be keeping a close watch on how it is used by venues and reserve the right to mandate if necessary.”
He continued: “By the end of September everyone aged 18 and over will have the chance to receive full vaccination and the additional two weeks for that protection to really take hold.
“So at that point we plan to make full vaccination a condition of entry to nightclubs and other venues where large crowds gather.
“Proof of a negative test will no longer be sufficient.”
Sir Patrick also warned that nightclubs could be “potential super spreading events”.
He said: “Right the way across the world we’ve seen that nightclubs and venues where you’ve got lots of people indoors, crowded together, are a focus for potential super spreading events, and that has also been seen in terms of what’s happened in Holland and Israel where nightclubs opened, and you saw a big increase in cases.
"So I think it’s... there’s no question that that is an environment in which spreading is easier, you’ve got lots of people quite close together, you’ve got the environment in which spreading becomes easier."
Watch: COVID-19: What are the remaining rules in England after 'Freedom Day'?