The UK's test and trace system for COVID-19 is currently inadequate for schools to reopen safely after the summer, a study suggested.
Only certain groups of children, such as those of key workers, Year 6 pupils, and some secondary school children approaching exams, have been back to school since late March.
The Government has said it wants to see all children in England returning to the classroom in September.
The move could help the struggling economy by freeing up parents to return to work.
The Department of Health and Social Care (DHSC) said test and trace capacity was growing.
Modelling of Six Different Scenarios
Researchers at University College London and the London School of Hygiene and Tropical Medicine said there was a lack of evidence on transmission of the SARS-CoV-2 virus in schools, with most existing data based on modelling and very few studies using real-world data from schools to investigate outbreaks.
They said they had given the first estimates on the levels of testing, tracing, and self-isolating that would be needed for schools and society at large to reopen while avoiding a second wave of the pandemic in the UK.
The study in The Lancet Child & Adolescent Health journal modelled six different scenarios of school reopening. These included full time schooling, and a part-time rota system with half of children attending school on alternate weeks. They also modelled three different types of test and trace regimes.
The models also included a relaxation of lockdown measures across society at large which it was assumed would have happened once schools reopened.
The study suggested that a second wave in the UK might be avoided if testing increased to include between 59% and 87% of symptomatic people, coupled with effective contact tracing and isolation.
For effective contact tracing and isolation, assuming 68% of contacts could be traced:
- 75% of individuals with symptomatic infection would need to be diagnosed and isolated if schools return full-time in September
- 65% if a part-time rota system were used
If only 40% of contacts could be traced, these figures would need to increase to 87% and 75%, respectively.
The researchers warned that if levels of diagnoses and contact tracing fell below these figures, reopening of schools together with gradual relaxing of the lockdown measures were likely to result in a secondary wave that would peak in December, 2020 if schools opened full-time in September, and in February, 2021 if a part-time rota system was adopted in September.
Test and Trace 'Is Working': DHSC
A spokesperson for the DHSC said: "NHS Test and Trace is already working - last week over 80% of those testing positive were reached with over 75% of their contacts reached as well.
"We have rapidly built, from scratch, the largest diagnostic testing industry in British history.
"Over 2.6 million people have been tested in just 8 weeks and we have the capacity to carry out more than 330,000 tests per day, growing to 500,000 per day by the end of October."
Commenting on the modelling study to the Science Media Centre, Martin Hibberd, professor of emerging infectious disease at the London School of Hygiene & Tropical Medicine, said that "a clear message is that contact tracing needs to be effective in order to control the number of infections as contact rates increase with school openings", and that "The evidence from the UK system of track and trace is not yet clear on whether it will be effective enough to play this role."
Last week, Prof Chris Whitty, England's chief medical officer warned there might need to be a trade-off between the Government's priority to reopen schools and opening the rest of society.
Matt Keeling, professor of populations and disease at the University of Warwick, said the paper was "an important addition to the debate around how the country can safely reopen".
The Lancet Child & Adolescent Health: Determining the optimal strategy for reopening schools, the impact of test and trace interventions, and the risk of occurrence of a second COVID-19 epidemic wave in the UK: a modelling study. Aug 3, 2020. DOI:https://doi.org/10.1016/S2352-4642(20)30250-9
