Children younger than five carried major amounts of coronavirus in their upper respiratory tract, a small study published on Thursday showed, raising new questions about whether kids can infect others.

Data on children as sources of coronavirus spread are sparse, and early reports did not find strong evidence of children as major contributors to the deadly virus that has killed 669,632 people globally.

Understanding the transmission potential in children will be key to developing public health guidelines, said the researchers who published the study in the journal JAMA Pediatrics.

Between March 23 and April 27, 2020, a research team from Ann & Robert H. Lurie Children’s Hospital and Northwestern University tested swab collections from inpatient, outpatient, emergency department, and drive-through testing sites in Chicago, Illinois.

The study included 145 individuals aged between one month and 65 years with mild to moderate COVID-19 who were studied in three groups – children younger than five years, children 5 to 17 years, and adults 18 to 65 years.

Their analysis suggests the young children had a viral load 10-fold to 100-fold greater than adults in their upper respiratory tracts.

Viral loads in older children with COVID-19 are similar to levels in adults. This study found greater amounts of viral nucleic acid – the genetic codes for proteins to produce new viruses – in children younger than 5 years.

The study only looked at viral nucleic acid and not infectious virus, meaning it is not clear if the children would spread the virus.

Still, the prevalence in young children raise concerns about their behavioral habits, and their proximity in schools and day care centers as public health restrictions are eased, researchers said.

In addition to public health implications, the researchers said the results could help put the focus on this population while targeting immunization efforts when COVID-19 vaccines become available.