Younger people aged 10 to 19 years old are more likely than other age groups to spread the coronavirus in their household, according to a large contact tracing study in South Korea soon to be published by the Centers for Disease Control, a concerning sign as U.S. school districts weigh whether to reopen for in-person classes in the fall.
The study is an early release of a forthcoming article in Emerging Infectious Diseases, a peer-reviewed journal published by the CDC.
Researchers followed 5,706 coronavirus patients from January 20 to March 27—when schools in South Korea were closed—who were the first to report COVID-19 symptoms in their household and traced all of their contacts to determine how the virus spread.
The study found that young people, between the ages of 10 to 19, were not often the first in their household to show symptoms—but when they were, 18.6% of their contacts contracted the disease, which is more than any other age group.
By contrast, children 9 years old and younger were the least likely group to spread coronavirus in their household, with 5.3% of their contacts—which represents three people—testing positive.
The second-most likely age group to spread the coronavirus in their household are older adults 70-79, who had 18% of their household contacts become infected (60-69 year olds follow with 17%).
Outside the household, older people between the ages of 70 and 79 were the most likely to spread the disease, with 4.8% of their non-household contacts becoming infected.
The authors say the study has limitations, such as the fact that all asymptomatic cases may not have been identified, but Dr. Ashish Jha, director of the Harvard Global Health Institute, told the New York Times that this is “one of the best studies we’ve had to date on this issue.”
“We showed that household transmission of SARS-CoV-2 was high if the index patient was 10–19 years of age,” the study says. “… The role of household transmission of SARS-CoV-2 amid reopening of schools and loosening of social distancing underscores the need for a time-sensitive epidemiologic study to guide public health policy.”
The study comes as the issue of school reopenings turns into a contentious political battle. The Trump administration is pushing for schools to reopen for in-person classes, and the president has criticized the CDC for proposing stricter reopening guidelines while blocking director Robert Redfield from testifying in Congress about the matter. The largest school districts are forging ahead with their own decisions, with New York City and Fairfax County, Virginia schools opting to partially reopen. Los Angeles and San Diego school districts, meanwhile, will be fully online.
Countries that have opened up schools have had mixed outcomes. Finland and Denmark have reopened schools without leading to large outbreaks, although the virus was more under control in those countries than the U.S. Reopened schools in South Korea and Israel, however, have led to a surge in cases.
Read the full study here.