Among children aged ≤ 6 months included in the ENVIE study, the ENVIE_ancillaire study will be offered to cases of RSV infection or to controls. The study involves a nasopharyngeal swab using a FLOQ-SWAB™ swab and a fingertip sample (dextro type). These samples will be collected specifically for the purpose of studying immunity.

A blood sample may also be requested for children weighing ≥5 kg who are included in the ENVIE_ancillaire study. The purpose of this sample collection will be to conduct an advanced immunological study

Among children aged ≤ 6 months included in the ENVIE study, the ENVIE_ancillaire study will be offered to cases of RSV infection or to controls. The study involves a nasopharyngeal swab using a FLOQ-SWAB™ swab and a fingertip sample (dextro type). This sample collection will be carried out specifically for the study of immunity. Innate immune mediators (type I, II and III IFNs) and key inflammatory molecules (IL-6, TNF-alpha, IL-10) will be quantified, and adaptive immunity will be analysed by measuring local immunoglobulin titres (IgG, A, M, subclasses and specific antibodies against the inactivated virus) from the nasopharyngeal swab and blood samples absorbed onto filter paper. A blood sample may also be offered to children weighing ≥5 kg who are included in the ENVIE_ancillaire study. The purpose of this sample collection will be to conduct an advanced immunological study (identification of the host's protein, cellular and transcriptomic response). Finally, the bacterial microbiome (16S RNA) and the soluble virome will be characterised by high-throughput sequencing.

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.

Among children aged 6 months or younger included in the ENVIE study, the ENVIE_ancillaire study will be offered to those with RSV infection or to the control group.