Scientists have cautioned against the reopening of schools after findings suggested children could be as infectious as adults.

The study, which was carried out by the team of leading German virologist Christian Drosten, found that even though children tended to have far milder symptoms, those infected appeared to have the same levels of circulating virus in their body as adults. This suggests schools and nurseries could act as hubs of Covid-19 transmission if current restrictions are lifted.

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“We have to caution against an unlimited reopening of schools and kindergartens in the present situation, with a widely susceptible population and the necessity to keep transmission rates low,” Drosten and colleagues concluded. “Children may be as infectious as adults.”

The study’s findings came as Unicef warned that the low number of vulnerable and disadvantaged pupils attending UK schools was “deeply concerning”, raising the risk of their education and safety being badly affected by the coronavirus crisis – particularly if schools are seen as hotbeds of infection.

Department for Education figures show just one in 10 pupils classed as vulnerable – those in care, with a social worker or with certified special needs or disabilities – went to school in England last week, despite being encouraged to do so.

Unicef’s views were echoed by the UK’s Equality and Human Rights Commission (EHRC), which issued a stern warning that GCSE and A-level grade assessments this summer should not unfairly penalise children from minority ethnic backgrounds, as well as disabled pupils and those with special educational needs.

Sacha Deshmukh, the executive director of Unicef UK, said that with more than 90% of vulnerable pupils not attending schools, British governments needed to think carefully about how they should receive greater support when schools are able to reopen.

“Low school attendance of vulnerable children is deeply concerning. School provides more than an education – for many it is their only safe space.

Q&A What are the UK government's 'five tests' for ending lockdown restrictions? Show Hide In April the UK government set out these five tests it said had to be met before they would consider easing coronavirus lockdown restrictions: The NHS has sufficient capacity to provide critical care and specialist treatment right across the UK

A sustained and consistent fall in daily deaths from Coronavirus

Reliable data to show that the rate of infection is decreasing to manageable levels across the board

Operational challenges including testing and personal protective equipment (PPE) are in hand with supply able to meet future demand

Confident that any adjustments to the current measures will not risk a second peak of infections that overwhelms the NHS

“We know that in the UK, continued closures are likely to widen inequality gaps, with the poorest and most vulnerable children expected to suffer ‘learning loss’ and have lower education attainment. This will have far-reaching implications on their futures,” Deshmukh said.

But policymakers will be forced to balance those concerns against the immediate risks that could be attached to reopening schools if children are just as infectious as adults. The German study, which was published as a preprint that has not yet been peer reviewed, screened nearly 60,000 patients for Covid-19, of whom nearly 4,000 tested positive. When the team compared the viral load across age groups, they found similar levels throughout, ranging from one to 10 years to 91 to 100 years.

There has been continuing speculation about when schools might reopen, and whether older year groups, including pupils in their final year of primary school and those in the middle of GCSE and A-level courses, could be among the first to be brought back into schools as they might benefit most.

When questioned at a select committee hearing on Wednesday, the education secretary, Gavin Williamson, declined to give a date for reopening, saying schools would open in a phased manner.

“We recognise that the idea of schools all returning on day one with the full complement of pupils is not realistic or practical,” he said. “I also intend to be giving schools as much notice as possible.”

As countries across Europe move to ease lockdown restrictions, there is growing discussion about the role of schools in boosting transmission. However, studies looking at the actual rates of transmission have been complicated by the fact many schools are currently shut or open only to small numbers of children. Some contact tracing studies suggest infection rates are similar in children to adults, while other studies have found little evidence of transmission in children.

“This lack of clarity on transmission produces uncertainty for planning to reopen schools,” said Prof Russell Viner, the president of the Royal College of Paediatrics and Child Health, who was not involved in the latest research. “Both keeping schools closed and reopening them carry degrees of risk in terms of population harms. Further data on susceptibility and transmission from population-based studies is urgently needed.”

Fewer children have been picked up in national testing programmes, due to milder symptoms. And during the early phase of the epidemic in Europe, adult travellers played a dominant role in seeding infections, which also meant, purely for circumstantial reasons, that children were more likely to catch Covid-19 than to spread infections to other household members. “This observation may be misunderstood as an indication of children being less infectious,” the authors said.

Quick guide Will there be a second wave of coronavirus? Show Hide In recent days the UK has seen a sudden sharp increase in Covid-19 infection numbers, leading to fears that a second wave of cases is beginning. Epidemics of infectious diseases behave in different ways but the 1918 influenza pandemic that killed more than 50 million people is regarded as a key example of a pandemic that occurred in multiple waves, with the latter more severe than the first. It has been replicated – albeit more mildly – in subsequent flu pandemics. Until now that had been what was expected from Covid-19.

How and why multiple-wave outbreaks occur, and how subsequent waves of infection can be prevented, has become a staple of epidemiological modelling studies and pandemic preparation, which have looked at everything from social behaviour and health policy to vaccination and the buildup of community immunity, also known as herd immunity. Is there evidence of coronavirus coming back in a second wave? This is being watched very carefully. Without a vaccine, and with no widespread immunity to the new disease, one alarm is being sounded by the experience of Singapore, which has seen a sudden resurgence in infections despite being lauded for its early handling of the outbreak. Although Singapore instituted a strong contact tracing system for its general population, the disease re-emerged in cramped dormitory accommodation used by thousands of foreign workers with inadequate hygiene facilities and shared canteens. Singapore’s experience, although very specific, has demonstrated the ability of the disease to come back strongly in places where people are in close proximity and its ability to exploit any weakness in public health regimes set up to counter it. In June 2020, Beijing suffered from a new cluster of coronavirus cases which caused authorities to re-implement restrictions that China had previously been able to lift. In the UK, the city of Leicester was unable to come out of lockdown because of the development of a new spike of coronavirus cases. Clusters also emerged in Melbourne, requiring a re-imposition of lockdown conditions. What are experts worried about? Conventional wisdom among scientists suggests second waves of resistant infections occur after the capacity for treatment and isolation becomes exhausted. In this case the concern is that the social and political consensus supporting lockdowns is being overtaken by public frustration and the urgent need to reopen economies. However Linda Bauld, professor of public health at the University of Edinburgh, says “‘Second wave’ isn’t a term that we would use at the current time, as the virus hasn’t gone away, it’s in our population, it has spread to 188 countries so far, and what we are seeing now is essentially localised spikes or a localised return of a large number of cases.” The overall threat declines when susceptibility of the population to the disease falls below a certain threshold or when widespread vaccination becomes available. In general terms the ratio of susceptible and immune individuals in a population at the end of one wave determines the potential magnitude of a subsequent wave. The worry is that with a vaccine still many months away, and the real rate of infection only being guessed at, populations worldwide remain highly vulnerable to both resurgence and subsequent waves. Peter Beaumont, Emma Graham-Harrison and Martin Belam

The latest work aimed to provide an indirect insight into infectivity. It is possible, the authors concluded, that because asymptomatic children are not coughing they would be less infectious, but the close physical contact between schoolchildren might compensate for that.

Prof Martin Hibberd, of London School of Hygiene and Tropical Medicine, said: “This new research is consistent with the concept that children can be infected and perhaps also transmit the virus, just as easily as other age groups, without suffering as many symptoms. With testing currently focussing mostly on those with COVID-19 symptoms, it may be difficult to quickly identify the full role of children in transmission.”

Unicef has published joint guidelines, with Unesco, the World Food Programme and the World Bank, on how local and national governments can best keep children safe when schools reopen.

Deshmukh said: “Reopening schools must be done with the safety and best interests of children at the heart of decision-making. We urge the UK government to use these guidelines as they consider a phased reopening of schools, and develop a comprehensive children’s recovery plan that outlines ongoing support for learners who may have fallen behind during school closures.”