The proportion of young people at secondary school to have had coronavirus has increased to one in 15, new figures show.
But while the number of people testing positive for the virus is estimated to have increased in England it has fallen in Scotland and Northern Ireland.
Around one in 70 people in private households in England had Covid-19 in the week to October 2, up from one in 85 the previous week, according to the Office for National Statistics (ONS).
One in 70 is the equivalent of about 786,300 people.
At the peak of the second wave in early January, around one in 50 were estimated to have coronavirus.
The ONS said rates have increased for those in school Years 7 to 11, people aged 35 to 49 and those aged 70 and over.
The percentage testing positive decreased for those in school Year 12 to age 24, while the trend was uncertain for all other age groups, it added.
Around one in 15 children in school Years 7 to 11 are estimated to have had Covid-19 in the week to October 2 – the highest positivity rate for any age group and up from one in 20 the previous week.
Kevin McConway, emeritus professor of applied statistics at the Open University, said that with around four million children in that age group in England, one in 15 equates to around 270,000 testing positive.
He said that based on separate ONS estimates on long Covid prevalence, this could mean about 13,500 could go on to have long Covid symptoms, and almost 7,000 will have symptoms that limit their day-to-day activity
Prof McConway added: “That’s rather a lot of children who will have their schooling and other activities disrupted.
“I think the main necessary action here it to get on with vaccinating those aged 12-15 as quickly as possible – though, given that it takes time for the vaccine to become effective, that’s still not going to bring down the infection rate in this group all that fast.
“I’m certainly not suggesting general school closures or anything like that, but I don’t think it’s an option to do nothing.”
Unison’s assistant general secretary Jon Richards said masks and school bubbles needed to be reintroduced and the vaccine rollout to 12 to 15-year-olds sped up.
He added: “The Government can no longer look the other way while coronavirus rates rocket in secondary school-age children and are still unacceptably high in primary.
“Pupils have faced more than a year and a half of disruption to their education and ministers must pull out all the stops to keep children in classrooms.
“These figures underline how the Government’s current approach is woefully inadequate.”
The ONS figures also showed that positivity rates for the over-70s are still low at one in 170, but this is up from one in 190 the previous week.
Sara Crofts, head of analytical outputs for the ONS Covid-19 infection survey, said: “There is a mixed picture of infection trends across the UK once again, with the largest increase seen in England.
“This has been largely driven by a notable increase among secondary school pupils, likely reflecting their return to school in September.”
The percentage of people testing positive for Covid-19 is estimated to have increased in all regions except the East of England where the trend is uncertain, the ONS said.
Yorkshire and the Humber and the East Midlands had the highest proportion of people of any region likely to test positive for coronavirus in the week to October 2, at around one in 55.
London and eastern England had the lowest at around one in 90.
In Wales, around one in 55 people were estimated to have had Covid-19 in the week to October 2, unchanged from the previous week and still the highest since the week to December 23 2020.
In Northern Ireland, the latest estimate is one in 130, down from one in 65 the previous week, and for Scotland it was around one in 60, down from one in 55 the previous week.