Children are more at risk of long-term harm if they do not attend school than if they return to the classroom despite coronavirus, the UK’s chief medical officers have warned.

In a joint statement issued ahead of the reopening of schools next month, the advisers said children have an “exceptionally low risk of dying” from Covid-19.

They said “very few, if any” children and teenagers would come to long-term harm from the virus solely by attending school, while there was a “certainty” of harm from not returning.

The chief and deputy chief medical officers said schools were not a “common route of transmission”, and that teachers were not at any increased risk of dying compared to the general working-age population.

However, they noted that data from UK and international studies suggested transmission in schools may be largely staff to staff rather than pupils to staff.

“This reinforces the need to maintain social distancing and good infection control inside and outside classroom settings, particularly between staff members and between older children and adults,” they said.

The advisers noted that reopening schools has not been usually followed by a surge in Covid-19 transmission but it could push the reproduction rate – the so-called R rate – above one.

If this happened it would require “local action and could mean societal choices” of imposing limitations on different parts of the community, they added.

Signatories to the consensus statement included England’s Professor Chris Whitty, Scotland’s Dr Gregor Smith, Wales’ Dr Frank Atherton and Northern Ireland’s Dr Michael McBride.

In other developments:

– Prof Whitty said it would be “foolish” to plan for winter on the basis of having a coronavirus vaccine, but said there was a “reasonable chance” there could be jabs available before the winter of 2021-2022.

– Education Secretary Gavin Williamson was facing fresh scrutiny after he reportedly cancelled a key meeting to go on holiday in Scarborough the week before the A-level exams fiasco, according to The Sunday Times. It is understood he was visiting his parents and is said to have kept in contact with the Department for Education during the trip.

– Gillian Keegan, an Education minister, was criticised after sharing photographs of herself in France as the results drama unfolded. Mr Williamson appears to have ‘liked’ several of the pictures.

A Public Health England (PHE) analysis, published on Sunday, found there were 67 single confirmed cases, four “co-primary cases” (two or more linked cases diagnosed at the same time) and 30 outbreaks of Covid-19 in schools during June.

It said the majority of cases linked to outbreaks were in staff and warned that school staff needed to be “more vigilant for exposure outside the school setting to protect themselves, their families and the educational setting”.

The analysis also said further school closures may be necessary in regions with increasing community infection but this should “be considered only in extremis”.

Prof Whitty told reporters the joint statement was not guidance to parents but a laying out of the evidence of “things we know with confidence, the things that we think are probable and also some of the things we don’t know and making clear there is always some residual risk”.

He said that there was “clear” evidence that the chances of children dying from Covid were “incredibly small” and they were less likely to get severe illness and end up in hospital due to the virus.

“So the reason that is important to lay out is the chances of children catching Covid and then getting long-term serious problems as a result of it, solely due to going to school are incredibly small,” he said.

“They’re not zero, but they’re incredibly small.

“The chances of many children being damaged by not going to school are incredibly clear and therefore the balance of risk is very strongly in favour of children going to school because many more are likely to be harmed by not going than harmed by going, even during this pandemic.”

The warning came as Labour leader Sir Keir Starmer said Boris Johnson’s commitment to get all children back to school was at “serious risk” after a “week of chaos” over exam results.

He said the last two weeks have been “wasted clearing up a mess of the Government’s own making”, telling The Observer: “I want to see children back at school next month, and I expect the Prime Minister to deliver on that commitment.

“However, the commitment is now at serious risk after a week of chaos, confusion and incompetence from the Government,” he added.

“Ministers should have spent the summer implementing a national plan to get all children back to school. Instead, the last two weeks have been wasted clearing up a mess of the Government’s own making over exam results.”

Unison’s head of education, Jon Richards, said it was “vital” school staff should be able to wear face coverings.

“It’s still unclear why Government guidance won’t allow them, when they’re recommended for other workplaces,” he said.

Mr Johnson said earlier this month that getting all children back to school full-time in England in September is the “right thing for everybody”, insisting they are “safe” and “Covid secure”.

However, teachers, scientists, opposition politicians and the children’s commissioner for England Anne Longfield have previously called for improvements to testing before pupils return.