Scientists say there could be a "perfect storm" this winter as COVID-19 cases rise just as immunity levels start to wane for the elderly and clinically vulnerable.
Infections have averaged more than 40,500 per day over the past week and have risen by a third in a fortnight.
Last week, the Office for National Statistics estimated that one in 60 people in England had the virus - one of the highest rates seen the pandemic began.
The high numbers previously attributed to infections among schoolchildren have now spread to those in their 30s and 40s.
Dr Stephen Griffin, chair of the virus division of microbiology society at the University of Leeds, said the lack of restrictions for meeting indoors during the winter period means cases are likely to spread to the elderly and clinically vulnerable.
"Young people who aren't vaccinated are mixing without any mitigations in school," he said.
"Then you've got them passing on to their parents and families and extended families and then, of course, reaching the older generations."
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Although nearly 80% of eligible people have received two vaccine doses, studies have suggested protection from vaccines gradually wanes over time.
Neil Ferguson, a member of the government's Scientific Advisory Group for Emergencies (SAGE), said one factor influencing the UK's high case numbers was that it has relied heavily on the AstraZeneca vaccine.
"While that protects very well against very severe outcomes of COVID, it protects slightly less well than Pfizer against infection and transmission, particularly in the face of the Delta variant," he said.
Dr Griffin said it that the combination of rising cases over the winter period at a time when protection from vaccines is likely to be waning would create the "perfect storm".
The government said the winter months look "challenging" and that it was keeping a "close watch" on cases.
However, it said new infections were roughly in line with predictions and that the vaccine programme had "substantially" weakened the link between cases, hospitalisations, and deaths.
On Tuesday, the UK recorded 223 COVID-19 deaths within 28 days of a positive test, in the biggest daily jump since early March.
Millions of people are being offered booster shots to top up immunity levels, but epidemiologists say the programme is moving too slowly.
"It's critical we accelerate the booster programme," Prof Ferguson added.
Across Scotland and England, only four million booster doses have been administered, according to the latest data. More than half of the people eligible haven't yet received one.
Some experts say the recent sharp rise in cases will encourage people to get third doses in the coming weeks.
"It's a natural human response that we take action when perceived risk is higher," Dr Raghib Ali, senior clinical research associate at the University of Cambridge's epidemiology unit said.
"So I expect people's behaviour to change as cases go above 50,000 a day - with regards to vaccine uptake, face masks and social mixing."
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