• The polarizing issue of whether schools should stay open is far from clear cut, with the World Health Organization urging policymakers to be guided by a risk-based approach to maximize the benefit for the population.
• Further studies to understand the risk of infection in children and to assess the transmission of the age group are ongoing.

LONDON — The science around the link between children and the spread of the coronavirus is still evolving, but a growing body of evidence suggests that kids attending school do play an important role in community transmission.

The issue comes as the resurgent spread of Covid-19 has prompted most of Europe to adopt the strictest level of public health measures, with many countries in the region shutting schools as part of a broader plan to curb infection rates and alleviate the strain on already stressed health facilities.

The polarizing issue of whether schools should stay open is far from clear cut, with the World Health Organization urging policymakers to be guided by a risk-based approach to maximize the benefit for the population.

School closures have a clear negative impact on children's health, the United Nations health agency has warned, citing setbacks to children's education and development, family income and the overall economy.

At present, the exact timing of how long schools will need to remain a least partially closed, and when they may be able to fully reopen, remains to be seen.

Does shutting schools slow the spread?

A study by researchers at the Swiss Federal Institute of Technology in Zurich determined that Switzerland's decision to shut schools last March was responsible for cutting mobility by 21.6%.

The study, published Sunday and not yet been peer-reviewed, found school closures ranked third in reducing mobility nationwide — and therefore Covid transmission.

A ban on gatherings of more than five people was shown to be the most effective policy tool, cutting mobility by around a quarter, while the closure of restaurants, bars and nonessential shops caused people to reduce total trips by 22.3%.

The study estimated that a reduction of 1% in human mobility predicted a 0.88% to 1.11% reduction in daily reported Covid cases, underscoring the effectiveness of shutting schools to slow the spread of the virus. It analyzed telecommunication data consisting of 1.5 billion trips taken by Swiss residents from Feb. 10 to April 26.

"Studies from interventions across hundreds of countries across the globe have consistently shown that school closures are associated with a reduction in R, and openings with an increase," Dr. Deepti Gurdasani, clinical epidemiologist at Queen Mary University of London, told CNBC via email.

The "R" rate, or reproduction rate, refers to the average number of secondary Covid infections produced by a single infected person.

What measures can be put in place to reduce risk?

In the U.K., schools for the large majority of children in England, Scotland, Wales and Northern Ireland are expected to remain closed for the coming weeks. And Gurdasani said data within England has previously shown that Covid cases drop during points of school closure, such as during mid-semester vacations.

Referencing data from the U.K.'s Office of National Statistics, she added children ages 2 to 11 had been found to be twice as likely as adults to be patient zero in the household, while those 12 to 16 years old were seven times as likely to bring infection into the family home.

The ONS examined household data within England between April and November last year. It also found that, once infected, children ages 2 to 16 were twice as likely to infect household contacts when compared with people over 17.

"In light of this, it's crucial that governments introduce mitigatory measures in schools, including smaller class and bubble sizes, better ventilation and air filtration with monitoring, mask use, social distancing, hygiene and use of large empty spaces to reduce risk," Gurdasani said.

"Acknowledging the impact of schools on community transmission is crucial to ensuring we minimise risk of transmission within schools, as well as from schools into the community."

What's the risk to young people?

The European Centre for Disease Prevention and Control has said, where an epidemiological investigation has occurred, Covid transmission in schools has accounted for a minority of all Covid cases in a given country.

The EU agency highlights that most children do not develop symptoms when infected with Covid and if symptoms do develop, they tend to be a very mild form of the disease.

That's not to say cases of critical illness have not been reported for children. The WHO says children under age 18 represent approximately 8.5% of reported cases. And as with adults, preexisting medical conditions have been suggested as a factor for severe disease outcomes and admission to intensive care.

What does the ECDC recommend?

"If appropriate measures are implemented in schools to reduce the possibility for the virus to spread, it is unlikely that school settings will play a substantial role in transmission," the ECDC says on its website. "Moreover, schools are an essential part of society and children's lives."

The European health agency added: "Widespread school closures should be seen as a last resort, and only considered after other measures have been put in place within the community to contain the spread of the disease."

Late last month, a review by the ECDC found that the return of schoolchildren in mid-August did not appear to have been the driving force in the surge in cases observed in many EU states experienced during the fall.

Further studies to understand the risk of infection in children and to assess the transmission of the age group are ongoing.