As Quebec schools open, researchers tackle a big unknown: What role do children play in the spread of COVID-19?

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Every parent in Canada wants to know how likely children are to spread the novel coronavirus, but Montreal epidemiologist Kate Zinszer needs an answer more than most.

Her husband has cancer and is undergoing chemotherapy, which makes him vulnerable to the virus. Her six-year-old son, meanwhile, has apraxia, a neurological motor disorder that affects his speech, which makes him vulnerable to falling behind without formal schooling.

“It’s honestly a tough choice,” Dr. Zinszer said of keeping her son, Charlie, out of kindergarten to protect his father. “Is this a critical period of time where he should be in school and he’s not going to be in school? It’s a worry.”

As Quebec children outside of Montreal head back to their classrooms on Monday, Dr. Zinszer, a professor in the department of social and preventive medicine at the University of Montreal, is preparing to lead a study that she hopes will shed light on the role that children play in the transmission of SARS-CoV-2, the virus that causes COVID-19.

Although details are still being ironed out, Dr. Zinszer and her colleagues at the University of Montreal, Laval University, the Montreal Public Health Department and the National Public Health Institute of Quebec are aiming to start collecting samples of saliva or blood, drawn through finger-pricks, from 3,000 to 4,000 Quebec children as soon as next month.

The rate at which young children spread the coronavirus remains one of the big unanswered questions of the pandemic.

There is little doubt children are capable of transmitting the virus, but plenty of doubt about how often they actually do.

Figuring that out is taking on more urgency as several European countries tentatively reopen their schools and as Montreal – the hardest-hit city in Canada – looks to welcome children back on May 25.

“The bottom line is, in the observations made up until now, it seems that kids are not an important factor in community spread,” said Jesse Papenburg, a pediatric infectious-disease specialist and medical microbiologist at the Montreal Children’s Hospital. “That’s not to say they don’t spread it. They do. But it just seems to be at a much lower frequency than older kids and adults.”

If that holds true as more evidence pours in, then this coronavirus would be substantially different from influenza and other respiratory viruses, which pass easily among runny-nosed children and on to the adults who cherish their grimy hugs and sloppy kisses.

In one well-publicized case, a nine-year-old British boy living in France was infected with the coronavirus as part of an outbreak at a ski chalet in the French Alps in late January.

Despite attending three different ski schools while mildly ill, the boy seemed to have passed the virus to just one of 73 close contacts who underwent testing, according to a study published in the journal Clinical Infectious Diseases. Even his siblings escaped infection.

In the Australian state of New South Wales, preliminary data on students and staff in 15 primary and high schools found only two cases in which the virus was thought to have been passed on at school, even though researchers at the country’s National Centre for Immunization Research and Surveillance identified initial cases in nine students and nine teachers.

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“This is consistent with the overall low rate of disease seen in children worldwide and evidence of limited child to child and child to adult spread,” the New South Wales researchers said in an e-mail to The Globe and Mail.

Children also seem less likely to kick off chains of transmission at home, according to an analysis by Chinese and Australian researchers published March 30 on the preprint server MedRxiv. The study, which has not been peer reviewed, found that children were the index, or first, cases in only three of 31 household clusters of COVID-19 in five countries.

In Switzerland, the health ministry’s infectious-diseases chief, Daniel Koch, told a news conference in late April that it was safe for children under the age of 10 to briefly hug their grandparents because young children, who generally have mild symptoms, are unlikely to catch or spread the virus.

But the country’s National COVID-19 Science Task Force disagreed, publishing a policy brief that found the evidence on children and transmission was “insufficiently robust” to draw conclusions.

The Swiss task force is hardly alone. Germany’s top virologist warned against a wholesale reopening of schools after publishing a study that concluded children infected with the coronavirus had as much of the virus in their noses and throats as adults, which suggests they’re just as infectious.

Other critics say some early studies raise more questions than they answer. Did the nine-year-old boy in the French case spread SARS-CoV-2 to so few because he was co-infected with influenza and another virus? Was the early data from Australia’s schools skewed by the fact that, halfway through the data-gathering period, the premier of New South Wales encouraged parents to keep their children home from school?

“There are limitations that really, I think, prevent drawing conclusions [from the Australian data] to say that there’s no transmission in school or that it’s exceedingly low,” said Stephen Freedman, a pediatric emergency medicine doctor at Alberta Children’s Hospital in Calgary who is leading an international study on how COVID-19 affects children.

The simple reality, other studies point out, is that even if children are less likely than adults to spread the coronavirus – perhaps because their mild symptoms mean they cough and sneeze less – the sheer number of contacts they’ll have when schools reopen could make classrooms a hotbed of infection, particularly if physical distancing isn’t enforced.

To take one example, a new study in the Canadian Medical Association Journal found that school closings, along with other public-health measures such as cancelling mass gatherings, were consistently associated with reduced growth of COVID-19 epidemics.

Much like Dr. Zinszer and her colleagues in Montreal, researchers at Toronto’s Hospital for Sick Children and St. Michael’s Hospital are trying to get a better handle on the role children play in transmission. They have asked a subset of parents from a long-running study called TARGet Kids to swab themselves and their children weekly for coronavirus testing.

Catherine Birken, a pediatrician at SickKids and co-leader of TARGet Kids, said they’re also asking participants to fill out regular surveys about how they’re coping with school closings and physical distancing, which pose their own risks.

“We are very interested in the infectivity,” Dr. Birken said, “but it feels like a really large impact on children will be from the isolation.”

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