NEW YORK • In the heated debate over reopening schools, one burning question has been whether and how efficiently children can spread the virus to others.

A large new study from South Korea offers an answer: Children younger than age 10 transmit to others much less often than adults do, but the risk is not zero. And those between the ages of 10 and 19 can spread the virus at least as well as adults do.

The findings suggest that as schools reopen, communities will see clusters of infection take root that include children of all ages, several experts cautioned.

"I fear that there has been this sense that kids just won't get infected or don't get infected in the same way as adults and that, therefore, they're almost like a bubbled population," said University of Minnesota infectious diseases expert Michael Osterholm.

"There will be transmission," he said. "What we have to do is accept that now and include that in our plans."

Several studies from Europe and Asia have suggested that young children are less likely to get infected and to spread the virus. But most of those studies were small and flawed, said Dr Ashish Jha, director of the Harvard Global Health Institute.

The new study "is very carefully done, it's systematic and looks at a very large population", Dr Jha said. "It's one of the best studies we've had to date on this issue."

Other experts also praised the scale and rigour of the analysis. South Korean researchers identified 5,706 people who were the first to report Covid-19 symptoms in their households between Jan 20 and March 27, when schools were closed, and then traced the 59,073 contacts of these "index cases".

They tested all of the household contacts of each patient, regardless of symptoms, but tested only symptomatic contacts outside the household.

The first person in a household to develop symptoms is not necessarily the first to have been infected, and the researchers acknowledged this limitation.

Children are also less likely than adults to show symptoms, so the study may have underestimated the number of children who set off the chain of transmission within their households.

Still, experts said the approach was reasonable. "It is also from a place with great contact tracing, done at the point interventions were being put in place," said Associate Professor Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

Children under age 10 were roughly half as likely as adults to spread the virus to others, consistent with other studies. That may be because children generally exhale less air - and therefore less virus-laden air - or because they exhale that air closer to the ground, making it less likely that adults would breathe it in.

Even so, the number of new infections seeded by children may rise when schools reopen, the study authors cautioned.

"Young children may show higher attack rates when the school closure ends, contributing to community transmission of Covid-19," they wrote.

Other studies have also suggested that the large number of contacts for schoolchildren, who interact with dozens of others for a good part of the day, may cancel out their smaller risk of infecting others.

The researchers traced the contacts only of children who felt ill, so it is still unclear how efficiently asymptomatic children spread the virus, said Dr Caitlin Rivers, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

"I think it was always going to be the case that symptomatic children are infectious," she said.

"The questions about the role of children are more around whether children who don't have symptoms are infectious."

Dr Rivers was a member of a scientific panel that last Wednesday recommended reopening schools wherever possible for disabled children and for those in elementary schools, because those groups have the most trouble learning online.

She said the new study does not alter that recommendation. The study is more worrisome for children in middle and high school. This group was even more likely to infect others than adults were, the study found. But some experts said that finding may be a fluke or may stem from the children's behaviours.

These older children are frequently as big as adults and yet may have some of the same unhygienic habits as young children. They may also have been more likely than the younger children to socialise with their peers within the high-rise complexes in South Korea.

"We can speculate all day about this, but we just don't know," Dr Osterholm said. "The bottom line message is, there's going to be transmission."

NYTIMES