The number of coronavirus cases among secondary school-aged children in England have begun to rise, with those in the later years believed to be the main driver for the increase in infections.

While sixth-form students and adults aged up to 24 recorded the highest proportion of infections in England towards the end of September, cases have started to rise in the year 7 to 11 age group, where the oldest students are 16.

The upward trend in the latest figures from the Office for National Statistics shows that infections among year 7 to 11 pupils have risen steadily since the end of August, to 0.3%. In the next age group up, those from year 12 to the age of 24, nearly half a per cent are estimated to have the virus.

“Cases have risen fastest in the 12- to 24-year-old age group. As a group, this age range is among the least likely to suffer serious illness from Covid but some of them will and will be left with life-changing complications,” said James Naismith, a professor of structural biology at the University of Oxford.

The analysis, based on nose and throat swabs taken from nearly 300 households across England, shows that infections among primary school children are flat or may even have declined since mid-September.

Prof Russell Viner, the president of the Royal College of Paediatrics and Child Health, who attends meetings of Sage, the government’s acientific advisory group for emergencies, said it was important not to blame uncontrolled socialising among young people for the upward trend in cases.

Those who have left school to live in shared houses may be particularly at risk, he said, and and young adults were most likely to have low-paid jobs that put them most at risk.

He said school outbreaks should also be put in perspective. “The evidence suggests that transmission in schools is very limited, even in secondary schools,” adding that “it is likely that much of the transmission among groups of young people may be outside school settings, as we really have limited evidence of transmission within schools”.

The ONS estimates that across England, 116,600 people had coronavirus in the week studied, from 18 to 24 September, up from 103,600 in the week before. That equates to about 8,400 new cases a day and about one in 500 infected.

Outbreaks in the north of England continue to drive the epidemic, with infection rates reaching 0.5% in the north-west and north-east. Substantial transmission in Yorkshire and the Humber has seen infection rates reach 0.3%, with infections in London close behind.

In its previous report, issued last week, the ONS estimated that in mid-September there were about 9,600 new infections per day in England. The figures suggest that while infections are still rising, the rate of growth may have slowed, though the ONS stressed that without more data it could not confirm this.

The statisticians are similarly cautious about hints of decline in Wales and Northern Ireland. This week, the ONS puts the proportion of people infected at one in 500 in Wales and one in 400 in Northern Ireland, compared with one in 300 for both regions in its last report.

“The rate of new infections definitely rose quite substantially during August and early September, and the new evidence indicates that this rise may be levelling off, though that isn’t yet by any means certain,” said Kevin McConway, the emeritus professor of applied statistics at the Open University.

According to Sage, the latest R value – the average number of people an infected person infects – stands at 1.3 to 1.6 across the UK, with the number of cases growing by 5% to 9% per day.

The figures are a marginal rise on last week, when the R value stood between 1.2 and 1.5 and the UK-wide growth rate was between 4% and 8%. In England, the latest figures from Sage put the R value at or above 1 in all regions. If R is above 1, cases have the potential to rise exponentially.

Given the time-lag in some of the data Sage use to calculate R, the figure tends to reflect the state of the epidemic in the preceding two or three weeks. This may explain why the numbers do not show any hint of the growth rate slowing, as found by researchers at Imperial College this week.