U.S. students are returning to school in person and online in the middle of a pandemic, and the stakes for educators and families are rising in the face of emerging research that shows children could be a risk for spreading the new coronavirus.

Several large studies have shown that the vast majority of children who contract COVID-19, the disease caused by the virus, have milder illness than adults. And early reports did not find strong evidence of children as major contributors to the deadly virus that has killed more than 780,000 people globally.

But more recent studies are starting to show how contagious infected children, even those with no symptoms, might be.

“Contrary to what we believed, based on the epidemiological data, kids are not spared from this pandemic,” said Dr. Alessio Fasano, director of the Mucosal Immunology and Biology Research Center at Massachusetts General Hospital and author of a new study.

Schools across the United States and elsewhere are trying out a wide range of strategies to reopen, from all online classes to all in person. They are asking whether reopening schools with stringent mitigation measures is worth the risk to students, families and educators, given that keeping schools closed will likely harm academic progress, social and emotional development, mental health and food security.

“We can’t just sit back and assume that there’ll be no problems with schools… I think we have to reopen schools, but we have to reopen them cautiously,” said Dr. Matthew Snape, Associate Professor in General Paediatrics and Vaccinology at University of Oxford.

Dr. Fasano and colleagues at Boston’s Massachusetts General and MassGeneral Hospital for Children found that infected children have a significantly higher level of virus in their airways than adults hospitalized in intensive care units for COVID-19 treatment. The high viral levels were found in infants through young adults.

The study, published on Thursday in the Journal of Pediatrics, involved 192 participants ages 0-22 who were seen at urgent care clinics for suspected COVID-19. Forty-nine of them - a quarter of the total - tested positive for the virus. Another 18 were included after being diagnosed with multisystem inflammatory syndrome, a serious COVID-related illness than can develop several weeks after an infection.

The research suggests that children can carry a high viral load, meaning they can be very contagious, regardless of their susceptibility to developing a COVID-19 illness.

“There has been some conflicting data out there about the degree to which children can be contagious,” said Dr. Marybeth Sexton, assistant professor of infectious diseases at Emory University School of Medicine in Atlanta, who was not involved in the study. “This is further evidence that we may see children as sources of infection.”

She added more extensive research is needed.

“NOBODY IS SPARED”

A separate study published last month in JAMA Pediatrics found that older children hospitalized with COVID-19 had similar levels of the virus in their upper respiratory tract as adults, but children younger than five carried significantly greater amounts.

However, other medical groups show differing information over children’s potential to spread the virus. The American Academy of Pediatrics on Wednesday updated its guidelines to reflect “that children under 10 years may be less likely to become infected and spread infection, while those 10 years and older may spread it as efficiently as adults.”

A recent South Korean study found that people were most likely to contract the new coronavirus from members of their own households, with children aged nine and under least likely to be the first identified case.

Since most children infected with the coronavirus have very mild symptoms, they were largely overlooked as a demographic in the earlier stages of the pandemic, Dr. Fasano said.

The U.S. Centers for Disease Control and Prevention reported a pediatric COVID-19 hospitalization rate of 8 per 100,000 for March 1 to July 25, compared with a rate of 164.5 per 100,000 for adults.

Experts say the incidence of a related disease, which can develop after COVID-19 infection, multisystem inflammatory syndrome, is concerning. “The number of these patients is growing,” Dr. Fasano added.

Concerns have also been raised about cases of type 1 diabetes among children diagnosed with COVID-19. A small UK study found that the rate of diabetes almost doubled during the peak of Britain’s COVID-19 epidemic, suggesting a possible link between the two diseases that needs more investigation.

“The more we understand, the more it boils down to nobody is spared in this pandemic,” Dr. Fasano said.