The return of English schools is likely to bring a “significant surge” in Covid-19 infections, a top epidemiologist has said.
Prof Neil Ferguson said it was too early to say whether fresh restrictions would be needed in order to contain such a surge.
The school holidays were regarded by ministers as a “firebreak” which would limit infections. Cases are high in Scotland, where pupils returned in August.
The UK government is spending millions of pounds on air quality monitors in classrooms in a bid to prevent further disruption.
Schools saw two prolonged periods of closure during the first and second waves of the pandemic in Britain.
Prof Ferguson, whose warnings in the early weeks of the pandemic led to the nickname “Professor Lockdown”, said the spread of the virus among mostly unvaccinated pupils could lead to strains on health care.
He said it was hard to predict how long any rise in infections would last after schools return.
“We expect to see quite a significant surge in cases, to some extent in hospitalisations,” said Prof Ferguson, a scientist at Imperial College London.
“Whether that’s going to require any rolling back of the relaxation of restrictions is too early to say. It really depends on the level of healthcare demand.”
Vaccines have weakened the link between cases and hospital admissions, but a very high number of infections could still cause pressure, Prof Ferguson said.
Hospital admissions and deaths have risen slowly over the summer but are at much lower levels than during the peak last winter.
Cases are high in Scotland, but Prof Paul Hunter, a professor of medicine at the University of East Anglia, said there were signs of the surge starting to fade.
He said schools might not be the only explanation behind the rise in Scotland. Increased social mixing over the summer could also be to blame.
“There is still a lot of uncertainty over how this will pan out,” he said, of the return to schools in England.
“Personally I would be surprised if case numbers don’t start to drift upwards again towards the end of this month but I doubt that we will see a huge sustained surge.”
The vaccination programme was extended to 16 and 17-year-olds last month. Some vulnerable children aged 12 to 15 are also eligible.
England lifted nearly all restrictions on domestic life on July 19, scrapping limits on how many people could meet socially.
Schools are no longer expected to keep children in segregated “bubbles”, and face coverings are no longer advised in classrooms.
In an effort to improve ventilation, schools are getting carbon dioxide monitors to check whether air breathed out by children is leaving the classroom.
A high CO2 reading suggests poor ventilation. Experts say good ventilation can sweep away airborne virus particles and reduce transmission.
Unions and opposition parties say ministers have been slow to improve ventilation after it became clear that this would make schools safer.