Nirsevimab Effectiveness Against Severe Respiratory Syncytial Virus Infection in the Primary Care Setting.

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Objective: This study assesses the effectiveness of nirsevimab, a monoclonal antibody, in preventing medically attended respiratory syncytial virus-lower respiratory tract infections (RSV-LRTIs) in a large primary care network in Spain, in both overall and catch-up infants aged younger than 10 months.

Methods: The 2023-2024 immunization campaign with nirsevimab in Spain targeted all infants born after April 1, 2023. Those born after October 1 received it at birth in hospitals, whereas others received it through a catch-up program. The MEDIPRIM network of primary care centers recruited all infants with LRTI for RSV polymerase chain reaction testing and employed a test-negative design approach to estimate the effectiveness of nirsevimab.

Results: The study included 160 infants; 141 (88%) of them received nirsevimab and 128 belonged to the catch-up group (88% received nirsevimab). Overall, RSV was detected in 44 infants (27.5%). Within the catch-up group, 37 (28.9%) were positive for RSV. The overall effectiveness was 75.8% (95% credible interval: 40.4-92.7), and 80.2% (95% credible interval: 44.3-95.4) in infants belonging to the catch-up group.

Conclusions: This study underscores the effectiveness of nirsevimab in preventing medically attended LRTI in infants in outpatient settings and emphasizes the importance of a catch-up immunization program to reduce the disease burden in primary care.