Effectiveness of Immunization in Preventing Severe Acute Respiratory Infection RSV in Children Under 6 Months in Bogotá

Respiratory Syncytial Virus (RSV) is a leading cause of hospitalization, morbidity, and mortality among infants, particularly during the first months of life. In 2026, Bogotá will introduce a hybrid immunization strategy combining maternal vaccination with RSVpreF during weeks 28-36 of pregnancy and neonatal immunization with nirsevimab for infants whose mothers were not vaccinated. Despite evidence from clinical trials, real-world comparative effectiveness data for hybrid RSV immunization strategies in Latin America remain limited.

This observational study will employ a prospective test-negative case-control design using the city's sentinel surveillance network for severe respiratory infection. All eligible infants under six months hospitalized for severe acute respiratory infection will undergo laboratory testing for RSV. Cases will be defined as RSV-positive infants, and controls as RSV-negative infants. Exposure to RSVpreF or nirsevimab will be validated through Bogotá's immunization registry.

Primary outcomes include laboratory-confirmed RSV-associated hospitalization, admission to intensive care, and death within the first six months of life. Multilevel logistic regression models adjusted for relevant confounders will be used to estimate vaccine effectiveness. Findings will provide rigorous real-world evidence to support policy decisions, optimize resource allocation, and guide the expansion of hybrid RSV prevention strategies in Colombia and other middle-income countries.

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 included).
• Delayed sampling: First respiratory sample obtained >48-72 hours after initial admission to the health system.
• Lethal congenital conditions: Congenital or genetic disorders incompatible with life or that substantially alter the expected clinical course.
• Concurrent interventional study participation: Enrollment in another clinical trial that may affect respiratory outcomes.
• Inability to establish mother-infant linkage due to adoption, loss of parental custody, or absence of biological maternal records.