Frequent respiratory infections could protect children from severe COVID-19...

Frequent respiratory infections could protect children from severe COVID-19…

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New Haven researchers may have discovered why children and adolescents were less likely to develop severe illnesses during the coronavirus pandemic than adults. The frequent respiratory infections that they contract, especially in early childhood, may have indirectly protected them, as the Journal of Experimental Medicine published study data suggest (2024; DOI: 10.1084/jem.20230911).

Children are generally more susceptible to respiratory infections than adults. Nevertheless, SARS-CoV-2 causes less severe disease in children than in adults, which is also reflected in the lower rate of hospitalizations and deaths in the COVID-19 pandemic.

It is known that the innate immune system in the nasal passages of children is more active than in adults. This may make it better able to fight off the early stages of SARS-CoV-2 infection. But the cause of this increased activity is not fully understood.

High immune activity in the child’s nose

Previous work suggests that the increased nasal innate immunity in children is due to intrinsic biological mechanisms associated with their age, reports senior author Ellen F. Foxman of the Department of Laboratory Medicine at Yale School of Medicine in New Haven. But we thought that the high burden of respiratory viruses and bacterial infections might also play a role.

Foxman’s research group analyzed 600 nasal swabs taken from children and adolescents during the COVID-19 pandemic. These were pediatric patients who either underwent elective surgery or were treated in an emergency room.

During the pandemic, the samples had only been tested for SARS-CoV-2, but Foxman and her colleagues tested them again, this time for 19 different respiratory viruses and bacteria. They also measured the levels of various cytokines (IL-1β, TNF, CXCL10) produced by the innate immune system.

Analysis for 19 different viruses and bacteria

They found that many children, even those without any symptoms, were infected with respiratory pathogens other than SARS-CoV-2. This was especially true for younger children: half of the asymptomatic children under 5 years of age were found to have representatives of the 19 viruses and bacteria examined.

Children who had more respiratory pathogens showed greater innate immune system activity in the nasal passages, regardless of whether they were toddlers or teenagers.

The detection of respiratory viruses correlated with CXCL10, whereas in bacterial infections a distinct proinflammatory response with increased IL-1β and TNF, but not CXCL10, was observed.

To further investigate the connection between respiratory infections and nasal innate immunity, the researchers compared nasal swabs from healthy 1-year-olds taken during routine examinations every 2 weeks.

More than half of the children tested positive for a respiratory virus at at least one of the two pediatrician visits. They had therefore either acquired an infection or recovered from an infection between the two appointments. In almost all cases, the children’s innate immune activity was higher when they were infected and lower when they were virus-free.

This shows that the nasal virus defenses in young children are not constantly on high alert, but they are activated in response to the invasion of a respiratory virus, even if the virus does not cause symptoms, Foxman said.

In summary, the study results suggest that the innate immune system in the nasal passages of children is often highly active because they are frequently infected with relatively harmless pathogens such as cold-causing rhinoviruses.

Foxman suspects that young children are more likely to be infected with seasonal viruses than adults because they are less protected by previous exposures. But because SARS-CoV-2 was a completely new virus to the human population, neither children nor adults had any existing protection when the pandemic began.

In this situation, the activation of generalized viral defenses in children by other infections may have helped to fight the early stages of SARS-CoV-2 infection, resulting in less severe COVID-19 courses than in adults. © nec/aerzteblatt.de

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