ENVIEANCILLAIRE: Efficacy of Nirsevimab in Children Hospitalised With RSV Bronchiolitis - A Subsidiary Study on the Immune Signature and Microbial Environment

Inclusion Criteria: * Written consent to take part in the study * Registered with a social security scheme Cases: * Children aged 6 months or under included in the ENVIE study. Treated for bronchiolitis caused by RSV * Admitted to hospital following a visit to the paediatric A\&E department Control group: Children aged 6 months or under included in the ENVIE study. Admitted to a standard ward or a Short-Stay Unit, or having attended the paediatric A\&E department For one of the following reasons: * Febrile urinary tract infection, without acute ENT or respiratory symptoms * Acute gastroenteritis, without acute ENT or respiratory symptoms * Infant colic without fever, without ENT or acute respiratory condition * Weight stagnation or feeding difficulties without fever, without acute ENT or respiratory symptoms * Neonatal jaundice without fever, without acute ENT or respiratory symptoms * Unexplained crying without fever, without acute ENT or respiratory symptoms * Head injury, without acute ENT or respiratory symptoms * Admitted for acute surgical conditions, without fever, without acute ENT or respiratory symptoms * Nappy rash, without fever, without acute ENT or respiratory symptoms * Fracture, without fever, without acute ENT or respiratory symptoms. Exclusion Criteria: * Refusal to participate by the legal representative * Administration of palivizumab * Maternal vaccination against RSV.