It comes as figures show most regions of England now have more coronavirus patients in hospital than at any point since mid-March.
Sixty percent of people being admitted to hospital with COVID-19 have had two doses of a coronavirus vaccine, according to the government's chief scientific adviser.
Speaking at a Downing Street news briefing, Sir Patrick Vallance said: "In terms of the number of people in hospital who've been double-vaccinated, we know it's around 60% of the people being admitted to hospital with COVID.
"We do expect there to be over 1,000 people per day being hospitalised with coronavirus because of the increase in infections.
"They're very, very effective, but not 100%, and 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 therefore will be amongst the people who both catch the infection and end up in hospital."
Two regions - south-west England and the combined area of north-east England and Yorkshire - are back to levels last seen more than four months ago.