Covid-19 infection levels among children of secondary school age in England have increased slightly, new figures suggest.

The percentage of children in school years 7 to 11 likely to have tested positive for Covid-19 in the week to March 20 is 0.43%, up from 0.32% the previous week.

The estimates, from the Office for National Statistics (ONS), coincide with the return from March 8 of pupils to secondary schools across England – a move that is likely to have affected the spread of coronavirus, thanks to the mixing of staff, parents and students.

Testing has also been scaled up, with all students in secondary schools expected to complete three rapid Covid-19 tests on their return, spaced three to five days apart, then further tests twice a week.

The ONS said rates are likely to have fallen among older teenagers and young adults (school year 12 to age 34) as well as those aged 50 to 69, but the trend is uncertain for other age groups.

All figures are for infections reported by people in private households, not other settings such as hospitals and care homes.

The estimates should be treated with caution, the ONS said, as they are based on small sample sizes and have a higher degree of uncertainty than the figures for England as a whole.

About one in 340 people in private households in England had Covid-19 in the week to March 20 – unchanged on the previous week, and the lowest figure since the week to September 24 2020.

The ONS said the overall percentage of people in England testing positive “is likely to have levelled off”.

Professor James Naismith, from the University of Oxford, said: “Today’s ONS data are less reassuring than last week; they serve as a caution but not yet evidence we need to change course.”

He added: “What has to be understood is that the new variants are harder to control and it seems likely that the limited reopening of schools has started to show up in data.

“Scotland has gone into this cycle first – its numbers levelled off around two weeks ago. Scotland should therefore be particularly closely watched with respect to decisions about further relaxation across the UK. If cases continue to rise in Scotland, it is safe to assume that this will happen everywhere else a week or two later.

“When is too early to relax social restrictions? The governments in the UK have made clear they will be guided by data not dates. I am afraid this is correct, frustrating as it is.

“We seem unlikely to lower prevalence further and may be lucky to keep at current levels, although more outdoor living will help as the weather improves.

“Realistically, we only have the vaccines as a way out. The successful vaccination campaign has already saved lives.

“Even in a full-blown third wave, the number of hospitalisation and deaths for every thousand people who become infected would be much lower because of vaccination.

“However, no one who has not been vaccinated is entirely safe from serious illness and a large wave of infection in young people will lead to hospitalisations.”

Lawrence Young, professor of molecular oncology at Warwick Medical School, added: “It was inevitable that infections would start to level off and even increase as we ease from lockdown.

“We are just beginning to see the impact of students returning to school, particularly in Wales and Scotland where schools went back earlier.

“Every change as we slowly release from lockdown is likely to have an effect on the levels of infection. This is why the road map is taking a cautious approach – leaving gaps between each relaxation so that the impact on infections can be carefully monitored.”