School outbreaks of Covid-19 may be due to the virus being taken into each school several times rather than transmission between pupils, a leading Public Health England (PHE) expert has said.

Dr Shamez Ladhani, a PHE consultant epidemiologist and chief investigator on a new study looking at coronavirus in schools, said experts are building a picture of how transmission happens, and whether schools introduce infection into the community or vice versa.

Revealed: School rapid Covid test areas in the South East

Headteachers: End Covid chaos and threats to schools

Wales: All secondary schools move to online learning

Early findings from a small study that he oversees suggest that the proportion of schoolchildren and teachers with coronavirus mirrors the proportion in the local community.

In a briefing with reporters, Dr Ladhani said it is clear there has been a week-on-week increase in infections in schools since early autumn.

He said the rate of infection goes up with age, with the highest rates in Years 11 (age 15-16) and 13 (age 17-18).

Coronavirus: ‘Little evidence’ that schools are driving the pandemic

But he added: “We know that there is infection happening in school-age children. What we don’t know is the dynamics of that infection - whether it is occurring in school or outside schools or at home.

“What we need to do more is try to understand how much of the infections are occurring through transmission within schools and how much of the infections are occurring outside schools, so that we can try and identify measures to mitigate this transmission to even lower levels.”

He said infection rates in schools could appear high in some regions because cases drop far more slowly among children than they do among adults.

PHE outbreak investigations have suggested there are different introductions of the virus into each school “rather than the virus being transmitted from a child in Year 1 to Year 3”, Dr Ladhani said.

“At the moment, a lot of the infections in schools and the outbreaks that you hear about does suggest they are different infections entering - sort of what we call multiple introductions - into schools, because they’re related to Year 1 and Year 4, Year 6 and Year 2, and so on.

“There’s not many outbreaks where six children in the same bubble, for example, all get infected in a timely way (where) you think that they passed it on to each other.”

However, he said the information “is a little sparse, and we need to do a lot more detailed investigation to try and answer those questions.”

Genetic studies to track the virus in schools are planned, he added.

In the new Schools Infection Study, released on Thursday, PHE, the Office for National Statistics and the London School of Hygiene and Tropical Medicine (LSHTM) sampled 105 schools and found similar levels of infection in schools as in the community.

Teachers and pupils were tested when they did not have any coronavirus symptoms - which means they were tracked in a normal daily school environment.

The data does not take account of pupils and teachers who were at home with the coronavirus or who were isolating because they were close contacts.

The data included 63 secondary and 42 primary schools in 14 local authorities, of which nine areas had high rates of Covid-19 and five had low rates.

Overall, 9,662 teachers and pupils took part in the study over several days in November.

The findings showed that 1.24 per cent of pupils and 1.29 per cent of staff overall tested positive for current infection - similar to the 1.2 per cent reported in the community.

In high prevalence areas, 1.47 per cent of pupils and 1.5 per cent of staff tested positive for current infection.

In low prevalence areas, this was 0.79 per cent of pupils and 0.87 per cent of staff.

Infection rates were higher in secondary schools compared with primary schools, but experts said this was not statistically significant.

Of the 105 schools surveyed, 47 (44.8 per cent) schools had no current infections, 29 (27.6 per cent) had one current infection and the remaining 29 (27.6 per cent) schools had between two and five current infections.

Dr Ladhani said: “While there is still more research to be done, these results appear to show that the rate of infection among students and staff attending school closely mirrors what’s happening outside the school gates.

“That’s why we all need to take responsibility for driving infections down if we want to keep schools open and safe for our children.”

Dr Ladhani said it is right, in his opinion, to prioritise keeping children in school.

“It’s not just about their education, it’s about their growth, it’s about their upbringing, it’s about their social skills, it’s about interacting with others, it’s about their mental health, it’s about making sure they get fed properly and they have access to social services,” he said.

“The list goes on and on and on and it is so difficult to measure, yet we know that it’s so important that we keep children in school, physically, so that there is some sort of normality in people’s lives.”

Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh, said: “We already know that schools are safe for the vast majority of children and that school staff are at no more risk than any other profession.

“I agree with the authors’ interpretation. Though there have been many Covid-19 cases in schools, there is little evidence that schools are driving the epidemic, as some have suggested they might.”