Effectiveness of Immunization in Preventing Severe Acute Respiratory Infection RSV

Inclusion Criteria: * Age: Infants younger than 6 months at the time of hospital admission. * Residence: Maternal residence in Bogotá, Colombia, verified through clinical record, identification document, or health system registry. * Clinical Condition: Hospitalization due to severe acute respiratory infection (SARI) meeting the district operational definition (presence of fever, cough, and/or signs of respiratory distress, requiring inpatient management). * Laboratory Testing: Respiratory specimen collected and tested for RSV by RT-PCR or validated antigen test within ≤48 hours of hospital admission, or repeated within 24-48 hours if initial test is negative. * Exposure Eligibility Window: Infant born ≥5 weeks after the start of the RSVpreF maternal vaccination campaign in Bogotá, ensuring opportunity for maternal immunization. Exclusion Criteria: * Prematurity: Infants born at ≤32 weeks of gestation. * Healthcare-associated respiratory infection: Symptom onset \>48 hours after hospital admission for another condition. * Non-respiratory hospital admission: Primary reason for hospitalization unrelated to respiratory disease (e.g., trauma, elective surgery). * Vaccination status unverifiable: Inability to confirm maternal RSVpreF vaccination status after reasonable attempts to verify through registry, vaccination card, or prenatal documentation. * Duplicate episodes: Readmission for the same respiratory episode ≤14 days after discharge (only the first episode will be include…