Covid-19: children less at risk of serious forms, really? | Press room

Covid-19: children less at risk of serious forms, really? | Press room

Reconstituted bronchial epithelium infected with Sars-CoV-2. On the left, the child, the infection is restrained. On the right of an adult, the infection spreads faster through the entire epithelium. © Harald Woodrich Laboratory

The start of the school year was accompanied by press articles on one of the topics that has been explored with great interest by the scientific community since 2020: children’s response to SARS-CoV-2 infection and vaccination. . In particular, are children really less likely to contract severe forms of Covid-19? And if so, why?

On these last two questions, Canal Detox reviews.

Since the beginning of the pandemic, many studies have been published on children’s ability to become infected. The research focused mainly on pollution dynamics in family homes, showing that children under the age of ten. On the other hand, teenagers were at the same risk of contamination as adults.

However, this data must be kept in mind with the fact that school children have on average more contacts than adults. In short, even if some data suggests that children are less likely to become infected with each encounter with the virus, they are in more contact and therefore exposed to it on average more than adults. This helps explain why the virus is spreading so aggressively in schools.

Furthermore, in relation to the risk of contracting severely from Covid-19 or of dying after infection, age. In addition, it was found that children, on average, have a low risk of developing severe forms of the disease.

different immunity

Research teams investigated the topic in order to provide explanations for this phenomenon, focusing on several hypotheses. It newly synthesizes it, in particular provoking a different immune response according to age but also differences in the intestinal microbiota that would protect children or even pre-existing immunity due to repeated exposure to other pathogens.

Some reminders about the immune response

Innate immunity is the immediate response that occurs locally, at the point of entry of a pathogenic microorganism, in an individual, even in the absence of prior contact with that microorganism. It is the first barrier of defense against pathogens. In the case of viral infections, they mainly include phagocytes (neutrophils, monocytes) and the natural killer that kills virus-infected cells. It also induces the production of interferon (including type III) by infected cells that protect neighboring cells from infection.

Adaptive immunity is a response that will take at least a week to be protective when encountering a pathogen for the first time (primary infection) but will be more effective when confronted with the pathogen already (we are talking about memory response). In the case of a viral infection, it involves two types of protective immune cells: antibody-producing B lymphocytes, which bind to the virus and “neutralize” it, that is, block its entry into cells and promote its elimination, and cytotoxic CD8 + T cells. Lymphocytes that kill infected cells. These responses are regulated by a third type of cell, CD4 T lymphocytes, which play a central role in the adaptive response. B and T lymphocytes recognize protein structures (from the virus) called “antigens”.

After a viral infection or vaccination, the rate of antibodies and lymphocytes that recognize the virus increases sharply before decreasing over time. However, the so-called “memory” B and T lymphocytes persist and patrol. They will act faster and more effectively during subsequent contact with the same virus, or after a booster dose.

Covid-19: children less at risk of serious forms, really? | Press room

If we are more specifically interested in the issue of immunity, which has been the subject of many studies, we can cite what made it possible to see things more clearly.

Using nasopharyngeal swabs from 226 people of different ages, the researchers showed that in people with SARS-CoV-2, the expression profiles of type I and type III interferon differ with age. Thus, children under 15 years of age had an increased expression of type III interferon, molecules that are not highly inflammatory and of local action, which control the virus at the level of its entry point, in the pharyngeal mucosa. On the contrary, adults, especially the elderly, express a preference for type I interferons, which are inflammatory and have a more systemic (throughout the body) action. These differences explain why children are less susceptible to critical forms than adults.

The role of the interferon response (particularly type III interferon) was also highlighted last June by the Inserm researcher team Harald Wodrich, in collaboration with groups led by Thomas Trian and Marie-Line Andreola. The scientists reconstructed the bronchial epithelium in the laboratory from samples from different donors including adults and children. Then they infected them with SARS-CoV-2. The scientists used imaging methods and virus estimation to track infections over time.

In general, they showed that the virus spreads rapidly in the bronchial epithelium. However, by comparing infections from different donors, they found that some epitheliums, especially those from children, were partially resistant to infection. This epithelium rapidly secretes type III interferon, which protects epithelial cells from SARS-CoV-2 infection.

After this work, the researchers confirmed the importance of the third type of interferon, by deleting the gene responsible for its production in this epithelium. They then showed that this led to a re-spread of the viral infection. Conversely, when the bronchial epithelium was treated with type III interferon, virus spread was controlled and the bronchial epithelium was partially protected from infection.

Future work will confirm these data and study in greater depth the differences in immune response from one child to another, but also the difference in the immunity of children compared to adults against other viral infections.

Keep caring about the long Covid

Research efforts regarding other factors that could explain why Covid-19 is generally less severe in children must continue. Work continues to continue driving for a person with postinfective myocarditis.

In the meantime, in the course of the long-running Covid studies, special attention can also be paid to the children and adolescents involved. While it is true that serious forms are less frequent in this population, and if prolonged Covid is considered rare, a publication in a scientific report that includes a sample of more than 80,000 children has raised some concerns.

She notes that 25% of children and adolescents with the infection will display one or more long-term symptoms, ranging from mood and sleep disturbances as well as extreme fatigue. Findings that justify this research continue to address the problem.

Written text supported by Research Director Harold WodrichInserm (Unit 5234 CNRS/Purdue University, Basic Microbiology and Pathogenesis) and Research Director Frederic Reux-Locatt Inserm at the Imagine Institute (Unit 1163 Inserm/University of Paris)

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