Respiratory tract infections caused by respiratory syncytial virus (RSV) are a worldwide burden and represent a major public health issue. In France, bronchiolitis is responsible for around 100,000 emergency room visits and 50,000 hospital admissions every year; 75% of infants hospitalised for RSV bronchiolitis are healthy full-term children.

Recent discoveries concerning the specific viral epitopes of RSV have made it possible to move from an empirical approach to a targeted preventive or curative approach (monoclonal antibodies, vaccines, anti-viral drugs).

Nirsevimab is a monoclonal antibody against RSV with enhanced neutralising activity and a prolonged half-life. A randomised, placebo-controlled phase III trial demonstrated the effectiveness of nirsevimab in reducing lower respiratory tract infections caused by RSV requiring medical management in healthy premature and term infants, with a favourable safety profile. The US Food and Drug Administration (FDA) approved the first RSV vaccine on May 3, 2023, and the second was approved on May 31, 2023. Nirsevimab was approved by the European Medicines Agency (EMA) on November 4, 2022.

Inclusion Criteria:

Cases :

• Children under 12 months of age
• Treated for acute RSV bronchiolitis
• Hospitalised following a visit to the paediatric emergency department

Controls :

• Children under 12 months
• Hospitalised in the conventional sector or in a short-stay hospital unit, or having consulted a paediatric emergency department for one of the following reasons
• Febrile urinary tract infection, without acute ear, nose, and throat (ENT) or respiratory symptoms
• Acute gastroenteritis, without acute ENT or respiratory symptoms
• Infant colic without fever, without ENT or acute respiratory symptoms
• Stagnant weight or feeding difficulties without fever, acute ENT or respiratory symptoms
• Neonatal jaundice without fever or acute ENT or respiratory symptoms
• Unexplained crying without fever, without ENT or acute respiratory symptoms
• Head injury, without acute ENT or respiratory symptoms
• Patient hospitalised for acute surgery without fever, without ENT or acute respiratory symptoms

Exclusion Criteria:

• Refusal to participate by the patient, their relative or legal representative
• Administration of Palivizumab.
• Maternal vaccination against RSV.

Paediatric patients under 12 months of age treated for RSV bronchiolitis (for cases), or for one of the conditions listed in the inclusion criteria (for controls).