Children, typically more prone to respiratory illnesses, experience milder cases of COVID-19 compared to adults, resulting in lower rates of hospitalization and death. Have you ever wondered why? Researchers have unraveled the intriguing secret behind their immunity.

As detailed in the latest issue of the Journal of Experimental Medicine, the key lies in children's heightened exposure to other infections. The frequent encounters with various viruses and bacteria seem to play a crucial role in shielding children from the severe impacts of COVID-19, boosting their immune responses in the process.

The innate immune system is the body's initial defense against viruses and bacteria. Research suggests that children have a more active innate immune system in their nasal passages compared to adults, potentially enhancing their ability to thwart early stages of SARS-CoV-2 infection.

The researchers of the latest study investigated if frequent respiratory infections are responsible for the heightened innate immunity in children.

"Prior work suggested that heightened nasal innate immunity in children was due to intrinsic biological mechanisms inherent to their age. But we thought it could also be due to the high burden of respiratory viruses and bacterial infections in children," Ellen F. Foxman, the senior author of the latest study, said in a news release.

In the study, the research team re-examined over 600 nasal swabs collected from pediatric patients during the pandemic. Initially, they were tested only for the SARS-CoV-2 virus before the patients underwent elective surgery or emergency room evaluation. These swabs were reassessed for 19 other respiratory viruses and bacteria. Additionally, the team analyzed the levels of antiviral and inflammatory proteins produced by the innate immune system.

A significant number of children, including those without any symptoms, tested positive for respiratory pathogens other than SARS-CoV-2. This was particularly seen among younger children, where viruses or bacteria-causing infections were found in approximately 50% of asymptomatic patients under the age of five.

"Children with higher levels of respiratory pathogens showed higher levels of nasal innate immune activity, regardless of whether they were toddlers or teenagers," the news release stated.

To further explore how respiratory infections affect nasal innate immunity, the research team studied nasal swabs from healthy one-year-olds at routine and follow-up pediatric visits. More than half of the children tested positive for a respiratory virus during one of the visits, suggesting they either contracted or cleared an infection in between. The researchers then observed that during infections, the children typically displayed higher innate immune activity and this decreased when they were virus-free.

"This reveals that nasal antiviral defenses are not continually on high alert in young children but are activated in response to acquisition of a respiratory virus, even when that virus is not causing symptoms," Foxman said.

"We have identified respiratory viruses and bacteria as key drivers of the enhanced nasal innate immunity in children. Our results compel further study of how seasonal respiratory viruses and nasal bacteria impact disease severity of COVID-19 and pediatric immune responses in general," Foxman added.