562. Maternal RSV Vaccine and Nirsevimab Effectiveness against Medically Attended RSV Disease in Infants, United States

Abstract Background During the 2023-2024 respiratory syncytial virus (RSV) season, two new RSV prevention products were recommended to protect US infants in their first RSV season, maternal RSV vaccine and nirsevimab. Using data from a 7-site respiratory virus surveillance network, we examined effectiveness of these products against medically attended RSV. Methods Infants with outpatient, emergency department, and inpatient visits for acute respiratory illness were enrolled during the 2023-24 and 2024-25 RSV seasons. All children had molecular testing for RSV and clinical and demographic data collected from caregiver interviews, medical records, and immunization registries. A test-negative design was used to estimate maternal RSV vaccine effectiveness (VE) in children < 6 months in both seasons and nirsevimab effectiveness in infants in their first RSV season in 2024-25. Cases and controls were infants who tested positive and negative for RSV, respectively. Maternal RSV vaccination was receipt >14 days before infant birth. Nirsevimab receipt was administration >7 days before symptom onset. Results Among 1248 infants < 6 months without nirsevimab receipt, 27/344 (8%) case and 144/895 (16%) control patients had maternal RSV vaccination; median infant age was 44 days (IQR 23-84). Maternal RSV VE was 63% (95% CI 40-77%) against medically attended RSV and 74% (95% CI 54-86%) against RSV hospitalization. Among 1362 infants in their first RSV season without maternal RSV vaccination, 60/452 (13%) case and 372/910 (41%) control patients received nirsevimab. Nirsevimab effectiveness was 78% (95% CI 69-84%) against medically attended RSV (median interval from receipt=62 days, IQR 35-96) and 82% (95% CI 72-89%) against RSV hospitalization; effectiveness was similar by RSV type and prematurity status. Nirsevimab effectiveness varied by time since receipt from 92% (95% CI 80-97%) in infants < 30 days from receipt (median interval=21 days, IQR 15-24) to 69% (95% CI 48-82%) in infants 90-164 days from receipt (median interval=111 days, IQR 102-126). Conclusion Under real-world conditions, both maternal RSV vaccine and nirsevimab effectively prevented medically attended RSV among infants in their first RSV season, with evidence of sustained nirsevimab protection through at least 5 months from receipt. Disclosures All Authors: No reported disclosures