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Background/Objectives: Respiratory syncytial virus (RSV) is the leading cause of infant hospitalizations in the United States. Prevention strategies are recommended to mitigate severe RSV outcomes. In addition to identifying potential coverage gaps, preventative administration timing is important for estimating product effectiveness. This study characterized administration timing of maternal and infant immunization against RSV across the United States during the 2024–2025 RSV season. Methods: A retrospective cross-sectional study was conducted using administrative claims of a commercially insured population from Kythera Labs. Pregnant individuals who received RSVpreF vaccine and infants who received nirsevimab were included. The seasonal cohort included infants born during the RSV season, while infants born from April to September were considered as the catch-up cohort. Baseline characteristics and calendar month and age at immunization (gestational age for RSVpreF) were evaluated. Results: Overall, 37,686 (71.9%) of maternal vaccinations were administered at 32–34 gestational weeks and 92.7% of all vaccinations occurred ≥14 days before delivery. Among infants who received nirsevimab, 34.8% of the seasonal cohort were immunized within 1 week of birth and 33.4% of the catch-up cohort were immunized in October 2024. Conclusions: Most maternal RSVpreF vaccinations occurred early in the recommended eligible gestational age window, while only approximately one-third of infants received nirsevimab during the first week of life or at the beginning of the RSV season. These findings highlight the importance of timely administration of RSV preventives. They further demonstrate that immunization timing should be incorporated into evaluation of the effectiveness and population level impact of RSV prevention programs.
A et al. (Mon,) studied this question.