Respiratory viral infections are among the most frequent diseases in humans. They represent significant burden on paediatric health. Indeed, children are the most frequently infected age group, with Rhinoviruses (RV) being the first cause of usually mild infection. Children are at higher risk of severe symptoms with other respiratory viruses, especially Influenza Virus (IV) and Respiratory Syncytial Virus (RSV). Children also significantly contribute to the propagation of these viruses in the general population. In 2019, SARS-CoV-2 emerged and caused a pandemic. Rapidly, concerns appeared about the impact of this new coronavirus on children. During the first year of the pandemic, it was observed that COVID-19 severity was lower in children than in adults. The contribution of the paediatric population to SARS-CoV-2 transmission and circulation in the general population was debated. I started my MD-PhD thesis in this context, willing to contribute to a better understanding of SARS-CoV-2 epidemiology and pathophysiology in children in comparison to other respiratory viruses. My MD-PhD thesis covers three aims: to estimate the SARS-CoV-2 shedding over time in children, adolescents, and adults and to establish if the infectious period varies with age (Aim 1); to study the transmission dynamics of SARS-CoV-2 in preschools and primary schools during the pandemic (Aim 2); and to investigate child and adult host response against SARS-CoV-2, Influenza A Virus (IAV), RV, and RSV in their first infection site, the nasal epithelium, and to understand if differences in the response could explain the milder clinical presentation of paediatric SARS-CoV-2 infection (Aim 3). I used various scientific approaches: retrospective analysis of clinical test results, epidemiological surveillance study in primary schools and preschools, and ex-vivo viral infection in an in vitro differentiated nasal epithelium model reconstituted from nasal brushes of children and adults. More than four years after the emergence of this coronavirus, the majority of the human population has been infected and/or vaccinated. However, a naive pool of infants will continuously appear. Young children might become one of the most impacted population categories, as this is the case with other endemic viruses. From this perspective, it is crucial to continue research on SARS-CoV-2 in children, notably to identify efficient and adapted measures to protect them against severe outcomes and to continue investigating the unique pathophysiology of SARS-CoV-2 infection in young individuals.