The magnitude of seasonal RSV epidemics brings each year new logistical challenges for the hospitalization of young infants with bronchiolitis that overwhelm hospital capacities and lead to specific winter plans with deprogramming and mobilization of human and logistical resources. The Covid-19 pandemic has changed the way winter epidemics are presented. For example, the seasonal RSV epidemic was shifted by several months in Lyon, with an impression of a lower incidence of hospitalized cases, with a population of older children and with fewer signs of clinical severity. This is largely attributable to the widespread use of barrier gestures and social distancing measures, known as "non-pharmacological interventions" or NPI. Given the magnitude of the reduction of the RSV epidemic, it is legitimate to analyze the benefits of NPIs to draw lessons for maintaining preventive measures around RSV-vulnerable populations; moreover, new preventive pharmacological interventions are soon to be marketed, whether they are particularly refined and long half-life anti-RSV monoclonal antibodies, RSV vaccines for mothers or for newborns and infants. In this perspective, it is crucial to properly define the populations at risk of severe disease to establish a legitimate hierarchy in the implementation of different preventive strategies. The study of the RSV epidemic is a high potential model because of the convergence of epidemiological, virological, and pharmacological knowledge. However, the study of the impact of the pandemic on the epidemiology of rhinovirus also seems promising because, for reasons unknown to date, it seems that the pandemic did not have the same reducing impact on the rhinovirus epidemic; in the latter case, the interest is to confirm the resistance of this virus and to look for more fundamental explanations, for example, on viral interactions.

On a previous study (see NTC 04944160), 519 infants and children were recruited in the Pre-Covid-19 season population, and 277 infants and children were recruited in the first Per-Covid-19 season population.

In the present study, the objective is to assess the epidemiology of RVS in infants from the birth cohorts of the tertiary teaching hospitals of Lyon, France, during the Pre-Covid-19 (2013-2020) and the Per-Covid-19 (2020-2025) years.

Inclusion Criteria:

• Child hospitalized at " Hôpital Femme Mère Enfant ", Lyon, France
• Positive RSV-PCR

Exclusion Criteria:

• • Parent's refusal to participate

: Children hospitalized in the Pediatric Department of the " Hôpital Femme Mère Enfant ", Lyon, France with a RT-PCR positive for RSV