Common colds boost children’s immunity against severe COVID

A team of researchers at Yale University has recently discovered that children’s frequent exposure to cold viruses and bacteria may have protected them from severe disease caused by SARS-CoV-2. 

According to the experts, common types of infections improve innate nasal immunity of young children so they can better confront the new coronavirus in earlier stages of infection.

Heightened nasal innate immunity 

Children are often more susceptible to various respiratory infections than adults, but during the pandemic, SARS-CoV-2 generally caused milder symptoms in children leading to fewer hospitalizations and deaths. 

The first line of defense, known as the innate immune system, appears to be more active in the nasal passages of children, thus hindering the early stage of coronavirus infection. Yet, the reasons behind this increased activity were previously unclear.

Study senior author Ellen F. Foxman is an associate professor of laboratory medicine and immunobiology at Yale. 

“Prior work suggested that heightened nasal innate immunity in children was due to intrinsic biological mechanisms inherent to their age,” said Professor Foxman. 

“But we thought it could also be due to the high burden of respiratory viruses and bacterial infections in children.”

Common colds and innate immunity in children

To assess whether frequent respiratory infections elevate nasal innate immunity in children, the experts re-screened over 600 nasal swabs collected during the pandemic from pediatric patients undergoing elective surgery or emergency room evaluations to test them for 19 different respiratory viruses and bacteria. 

During this process, they measured the levels of antiviral and inflammatory proteins produced by children’s innate immune systems.

The analyses revealed that many children – even those without symptoms – were infected with other respiratory pathogens. This was especially common among younger children, with around 50% of asymptomatic patients under five years old harboring various viruses or bacteria. 

Regardless of age though, children with higher levels of respiratory pathogens exhibited increased nasal innate immune activity.

Activation of viral defenses

To further understand the relationship between respiratory infections and nasal innate immunity, the experts compared nasal swabs from healthy one-year-olds at routine checkups and follow-up appointments one to two weeks later. 

Over half of the children tested positive for a respiratory virus during one of their two visits, indicating they had either acquired or cleared an infection in that period. Nearly every time, the child’s innate immune activity was higher during infection and lower when 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 explained.

The innate immune system in children’s nasal passages is frequently activated due to frequent infections with pathogens such as rhinoviruses, which cause common colds. 

The role of seasonal respiratory viruses 

According to Foxman, children have more infections with such seasonal viruses than adults because they lack immunological protection from prior exposures. 

During the COVID-19 pandemic, neither adults nor children had prior protection against the new coronavirus. Thus, the activation of generalized antiviral defenses in children by other infections may have helped them fight off early stages of SARS-CoV-2 infection, resulting in milder outcomes compared to adults.

“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 concluded.

The study is published in the Journal of Experimental Medicine.

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