Effectiveness of Immunization in Preventing Severe Acute Respiratory Infection RSV

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.