This study aims to evaluate the real-world effectiveness of two preventive immunization strategies against Respiratory Syncytial Virus (RSV)-associated severe acute respiratory infection in infants less than six months of age in Bogotá, Colombia. The strategies include maternal vaccination with RSVpreF administered between 28 and 36 weeks of gestation and neonatal immunization with nirsevimab for infants born to mothers who did not receive RSVpreF during pregnancy. Using a test-negative case-control design embedded in the city's sentinel surveillance system, infants hospitalized for severe respiratory infection will be systematically tested for RSV. Comparative vaccine effectiveness will be estimated to determine the impact of maternal RSV vaccination and neonatal monoclonal antibody immunization on RSV-associated hospitalizations, intensive care admissions, and mortality. The study will generate real-world evidence to inform local and regional public health decisions and guide the implementation of cost-effective hybrid immunization strategies against RSV in middle-income settings.

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.

Infants younger than six months residing in Bogotá, Colombia, hospitalized for severe acute respiratory infection in sentinel hospitals and tested for RSV within 48 hours of admission. The population includes infants exposed to maternal RSVpreF vaccination, neonatal nirsevimab, or neither intervention. Extremely preterm infants (≤32 weeks gestation) and those hospitalized for non-respiratory conditions are excluded.