Infant Respiratory Syncytial Virus Immunization Through Maternal Vaccination and Nirsevimab

Importance: In 2023, 2 forms of respiratory syncytial virus (RSV) prevention, maternal RSV vaccine and nirsevimab, became available for infants. The factors that played a role in their uptake during the first 2 seasons remain unclear. Objective: To describe the rate of RSV immunization through maternal RSV vaccination or nirsevimab during the 2023-2024 and 2024-2025 RSV seasons and identify the factors associated with their receipt. Design, Setting, and Participants: This cohort study was performed at clinical sites affiliated with a quaternary care hospital in New York, New York, from October 1, 2023, to March 31, 2024, and October 1, 2024, to March 31, 2025. Participants included infants younger than 8 months who had at least 1 clinical encounter during the 2023-2024 and 2024-2025 RSV seasons. Main Outcomes and Measures: The primary outcomes were receipt of RSV immunization by maternal RSV vaccine 14 days or more prior to delivery or nirsevimab. No evidence of RSV immunization was defined as either no documentation of either product or receipt of maternal vaccine less than 14 days prior to delivery without subsequent nirsevimab immunization. A multinomial logistic regression model was used to estimate odds ratios (ORs) controlling for age, sex, RSV season, insurance, and race and ethnicity, to assess factors associated with vaccination. Results: Of 13 195 eligible infants (6831 [51.8%] male; 8367 [63.4%] newborn; median age of nonnewborn infants, 17.7 [IQR, 8.1-26.9] weeks), 11 804 of 12 913 (91.4%) were born at term (gestational age ≥37 weeks), 11 208 of 12 964 (86.5%) were privately insured, and 12 109 of 13 195 (91.8%) were born at the study institution. A total of 8830 infants (66.9%) received RSV immunization through maternal RSV vaccine (3832 [29.0%]) or nirsevimab (4998 [37.9%]). RSV immunization coverage increased from 3595 of 6245 infants (57.6%; maternal RSV vaccine, 1317 [21.1%]; nirsevimab, 2278 [36.5%]) in the 2023-2024 season to 5235 of 6950 (75.3%; maternal RSV vaccine, 2515 [36.2%]; nirsevimab, 2720 [39.1%]) in the 2024-2025 season. Increased odds of receiving maternal RSV vaccine (adjusted OR [AOR], 3.58; 95% CI, 3.22-3.99) and nirsevimab (AOR, 1.89; 95% CI, 1.73-2.06) were associated with the 2024-2025 season compared with the 2023-2024 season. Lower odds of receiving maternal RSV vaccine (AOR, 0.18; 95% CI, 0.15-0.22) or nirsevimab (AOR, 0.80; 95% CI, 0.70-0.89) were associated with public compared with private insurance. Conclusions and Relevance: In this cohort study of eligible infants younger than 8 months, receipt of RSV immunization through maternal RSV vaccine or nirsevimab improved in the 2024-2025 compared with the 2023-2024 RSV seasons. However, infants with public insurance were less likely to receive either product, highlighting persistent disparities in RSV immunization warranting targeted interventions.